Driveshaft Length
It puts the nose of the water pump right in the area formerly occupied by the radiator and the hood latch mechanism. You can't move the radiator forward because that's where the grille and bumper is.
Using a truck oil pan requires moving the engine forward ~7" give or take, but the sump is 1 1/2 inches longer than the A body pan, so it will still require reworking anyways in order to maintain adequate K frame/steering linkage clearances. In addition, the main body (not the sump, the main body) of the truck pan is >1" deeper than the A body pan, which will require you to raise the engine higher in the engine bay in order to maintain proper clearance to the K frame- and depending on your manifold/carb choices, there goes all your hood clearance. Moving the engine forward also eliminates your ability to use the K frame for engine mounting, so you're stuck with buying/building an engine plate.
Your plan to use rear dump exhaust manifolds puts the exhaust dump right where your steering box currently is. Your choice of power steering only makes things even more restricted in an early A body, which is extremely cramped to begin with.
The 46RH transmission's mount is ~8ish inches further back than a standard automatic, so as far as the trans goes that's about how far you'd have to move it forward- but you'll STILL have to cut your crossmember, since there just isn't enough clearance for the overdrive extension housing; an early A just barely clears a standard tailshaft housing. And you can't just gain clearance by mounting the tailshaft lower by, let's say, 3". Remember- you already had to RAISE the engine an inch or more... can you even imagine how far out of whack your driveline angles are now? Not to mention ground clearance at the tailshaft/crossmember. So, you'll be cutting your crossmember anyhow in order to get the transmission high enough to get workable angles, which is going to be even more extreme than normal since you've raised your engine an additional inch.
Look closely at this pic of a standard 727 (center) flanked by two different OD transmissions:
Note where the transmission mount is located on the normal transmission (red arrow) versus the OD trannies (green arrows). That's about an 8" difference, as I recall. And note the diameter difference in the tailshaft housings; early As BARELY clear a normal transmission- that's why you need to cut the crossmember hoop (and a bit of tunnel massaging) to get the ODs to fit, no matter where you mount them. And yes, you still need to modify/fabricate a new center bolt-in section to accommodate the bigger OD cross-section and trans mount location no matter where you mount the trans.
And back to your original question: Why is a stock driveshaft length even relevant here? It doesn't apply no matter how you do the swap, with the engine & trans mounted anywhere from 8-10 inches forward and hanging out of the front end of the car with a block plate where the radiator yoke should be, or in the stock location with a few comparatively easy tunnel and firewall/wiper motor mods- you'll need a custom length shaft regardless.
FWIW, USCT and Holley both offer crossmember kits to make the work go easier (none specific to early As, but you've got to cut & weld anyway, and they're sized to give you the proper clearances and mounting angles).
Moving the engine forward does a number of things (other than destroying an already marginal front/rear weight bias):
It puts the nose of the water pump right in the area formerly occupied by the radiator and the hood latch mechanism. You can't move the radiator forward because that's where the grille and bumper is.
Using a truck oil pan requires moving the engine forward ~7" give or take, but the sump is 1 1/2 inches longer than the A body pan, so it will still require reworking anyways in order to maintain adequate K frame/steering linkage clearances. In addition, the main body (not the sump, the main body) of the truck pan is >1" deeper than the A body pan, which will require you to raise the engine higher in the engine bay in order to maintain proper clearance to the K frame- and depending on your manifold/carb choices, there goes all your hood clearance. Moving the engine forward also eliminates your ability to use the K frame for engine mounting, so you're stuck with buying/building an engine plate.
Your plan to use rear dump exhaust manifolds puts the exhaust dump right where your steering box currently is. Your choice of power steering only makes things even more restricted in an early A body, which is extremely cramped to begin with.
The 46RH transmission's mount is ~8ish inches further back than a standard automatic, so as far as the trans goes that's about how far you'd have to move it forward- but you'll STILL have to cut your crossmember, since there just isn't enough clearance for the overdrive extension housing; an early A just barely clears a standard tailshaft housing. And you can't just gain clearance by mounting the tailshaft lower by, let's say, 3". Remember- you already had to RAISE the engine an inch or more... can you even imagine how far out of whack your driveline angles are now? Not to mention ground clearance at the tailshaft/crossmember. So, you'll be cutting your crossmember anyhow in order to get the transmission high enough to get workable angles, which is going to be even more extreme than normal since you've raised your engine an additional inch.
Look closely at this pic of a standard 727 (center) flanked by two different OD transmissions:
Note where the transmission mount is located on the normal transmission (red arrow) versus the OD trannies (green arrows). That's about an 8" difference, as I recall. And note the diameter difference in the tailshaft housings; early As BARELY clear a normal transmission- that's why you need to cut the crossmember hoop (and a bit of tunnel massaging) to get the ODs to fit, no matter where you mount them. And yes, you still need to modify/fabricate a new center bolt-in section to accommodate the bigger OD cross-section and trans mount location no matter where you mount the trans.
And back to your original question: Why is a stock driveshaft length even relevant here? It doesn't apply no matter how you do the swap, with the engine & trans mounted anywhere from 8-10 inches forward and hanging out of the front end of the car with a block plate where the radiator yoke should be, or in the stock location with a few comparatively easy tunnel and firewall/wiper motor mods- you'll need a custom length shaft regardless.
FWIW, USCT and Holley both offer crossmember kits to make the work go easier (none specific to early As, but you've got to cut & weld anyway, and they're sized to give you the proper clearances and mounting angles).
