BB 727 to A518 Conversion

What speeds are you thinking of cruising?
It's a BB......roadtrip car......
Have you given thought to just putting hiway gears in the back?
Or maybe it already has 2.45s?These cruise at about 75=2280,and 65=1980 which is about the same as the A518 and 3.55s..
The 2.45 rear gears likes a lower first gear, like what's in the A500 and A998/999. But 2.45s and overdrive cruises at about 75=1580,which the BB can do, but 65=1380, which it might not like,in terms of fuel mileage, especially not a 40 year old engine/carb.
The next closest gear is a 2.76. With overdrive, that cruises 75= 1780; and 65=1540; still a bit low for the old low compression 400. Not too low that it can't do it, but too low for increasing fuel mileage.
All rpms are zero-slip.
There comes a point in the rpm band when lower rpm does not yield DEcreased fuel consumption. I don't know exactly where that is with a Lo-C 400.But I could guess at maybe 1800 PURE GUESS. If that were true, then to cruise 75@1800 would be, the above 2.76s and overdrive.But again at 65 you'd be out of the "zone".
Now, if you bumped up the compression, new story.
What hi-compression does is allow you to cruise on the top of transfers instead of on the bottom of the mains.
The TQ carb does really well on the bottom of the mains, as does the Q-Jet, and to some extent the AFB/AVS.These are all metering-rod carbs and you can tune that part of the fuel delivery pretty lean. But Holleys and their type are not so endowed.And the low-speed circuits on them are almost invariably rich, and nearly impossible to change.After all they have a racing heritage.
I once cruised an 11/1- 360 at 65=1590, and got phenomenally low fuel consumption with a Holley 600,leaned out specifically for the trip.
Timing is the big thing.
With falling rpm comes falling centrifugal timing, and falling vacuum can timing. Once you start cruising below 2000, getting adequate timing becomes very difficult, and so the engines efficiency takes a dump, and fuel-economy along with it. Timing can fall to say 18 to 20 in that range, where the engine may want 30 or 40,or more. And there is no way to give her what she wants, and still have her survive under power, short of computerized timing controls.
Without the timing, you have to drive deeper into the throttle, and that wakes up the mains.
The best solution is more compression. Lots more.
But that gets you into hi-test gas on the power circuit. And since the fuel bowls are usually common to both circuits,you have to run hi-test all the time, and so now it costs more dollars per mile because of that.
I tend to favor a minimum cruise rpm of 2000, with lots of Dynamic compression. To get 65=2000 you need 2.45s. Then crank in as much timing as she'll take. Then start leaning it out. The higher your compression ratio is, generally,the leaner you can make it.
2.45s usually mean lo-performance on take-off, and that is why those lo-po 318s have a 2.74 low gear. This gives a starter gear of 6.71. This is about the same as the 727s 2.45 low-gear and 2.76s. So putting 2.45s behind a trans with a 2.45 is like suicide, on take-off. Thankfully your 400 brings much-needed low-rpm torque to the party.Easily enough to pull that crummy 2.45 x 2.45 starter gear. But don't try that with a slanty,lol,like I did.
The GVOD would cost you well over $3000 installed.Probably closer to 4G.
For about the same, you could fix your compression.And slide a lil cam in there at the same time.Not enough to affect your low-rpm performance, but enough so that when you hit passing gear, it actually makes a difference.
I have never heard anyone say "my hi-compression engine sux".
If you keep the 2bbl, it's probably a metering rod carb,and that's a good thing as they are adjustable. And in this case, if it's a Holly it's gonna be pretty lean already. If the car has 2.45s or will get 2.45s, then that carb is almost guaranteed to be too-lean. Unless you crank in the timing, or crank up the compression. They are adjustable.
Happy HotRodding