5.9 Magnum 5-speed In an early A??

I believe this is a NV3500
Those gear-spreads are set up for a truck; not very good for a streeter. Low is much to low.
4.02-2.32-1.40-1.00-.73
The 1-2 split is 57.7%. What that means is when you typically out-shift first at 2800 ish, it drops into second at about 1600; so your engine better be able to pull at that rpm. Typically a performance engine kindof plays dead down there. So if your engine doesn't wake up until 2200, then first will have to wind up to 2200/.577=3800. Do you really want to wind it up that high in first nearly every time you go out.
Oh, and 2-3 ain't much better at 60.3%.
The 3-4 is pretty good at 71.4%
And overdrive is too close at 73%
This tranny works very well in it's original configuration, behind a torquey low-rpm engine.

I guess you could kindof make it work with say 3.55s, cuz then your starter would be 14.27, and second would be a kindof normal 8.23. But now 3rd is still out there. Still 3000 in second is almost 30 mph so you are kindof looking to merge with traffic anyway so into third it goes and you are cruising at 1800ish. While that is by itself prettygood.But your average LA performance teener hasn't got a lot of pull down there, so if you are heading out on the highway, you might have to keep in second until 4000, to drop into third at 2400. Oh, that hurts. So now you are at 2400 in third at 39mph, and it's still a long way to 65. But let's continue. The next gear is a little closer so unless you are driving a brick, lets shift a lil sooner, say 3700. Now you will enter 4th at 2700, a reasonable rpm, and you are moving at just about 60mph, Oh man, so close. 65 in 4th will be 2867, just 167 rpm, to go 6 mph.Kindof a short gear there,eh
So now you arrive at 65 and stuff it into o/d, and the Rs drop to 2035. Well surprise,surprise, that's the perfect cruiser gear for an LA teener! But notice that fourth was kindof a wasted gear.
If you swap in a different rear gear, to actually use that 4th gear (I mean you might as well use it if you got it, right?), you will find that 4.30s about covers it. These will get you 65@3472; and o/d will drop that to2465, still doable. But what happens at the other end? You guessed it the starter gear is now 17.29;a totally useless granny-gear. Second is now 9.98 a very nice teener starter gear.
No, 3.55s are the right rear-gear to run; the ratio spreads are just too far apart for an A-body V8.
Unless you need a crawler gear where 644rpm is 3mph. This is actually a good parade gear! Now you can hang with the automatics and have a clutch left when the parade is over.lol

OK so, wait! There is a Dakota version with a bit better gearing;
3.49-2.14-1.38-1.00-.73o/d with splits of .61-.64-.72-.73.
This is a bit better. But you are still gonna have to rev first to 3600 to catch second at 2200, and to 3438 to catch third at 2200, and to 3055 to catch fourth at 2200, and to 3014 to drop into o/d at 2200=68mph with 3.55s, and 27" tires.
And 3.55s will get you a nice 12.39 starter gear.
So build your teener to make some oats at 2200 and you will have some fun.

Hey, I might get me one of these, and put it behind a 273! But I think I'll use 3.23s or maybe 2.94s. Hyup, 2.94s will still get me a 10.26 starter, and a hi-way gear of 2.146;oh yeah!; mpgs here we come! 62@1652
I'm starting to like this closer ratio one!