The main difference in the 2bbl and 4bbl 400s is in the camshaft, exhaust manifolds, and dual exhaust. The factory TQuad has way too much airflow potential for th....
Even with the stock 2.71 rear axle ratio. Although a 3.21 might work better at lower speeds.
Try not to "over-think" this deal, which can be easy to do. Just a normal, stock rebuild, with the earlier closed chamber heads (as mentioned), a comparable cam, HP exhaust manifolds and a good under-car exhaust system are the basics you'll need.
All MUSIC to me! GOOD MUSIC! I LOVE my 2.76 rear end, even in the city. I just drive in 2nd gear on any thoroughfare posted under 45 mph and that old 383 still only turns over 2k rpm when passing or getting on the freeway, at which time I shift to 3rd/Drive once up to speed anyway. I plan to get a GOOD machine shop for the 915 heads I now have for my 400 and the forged crank from a 69 Commando 383. I prefer to use the forged crank, as I then can stay with the same internal balance setup and torque converter, though the cast crank with the TC and RV 727 I got w the 400 will do well enough, should there be some reason prohibiting the use of the forged crank.
Mind you, I'm not in any rush for replacing the engine I'm running! It idles at 500 rpm, pulling 17.5* inches vacuum, and while it leaks a little oil, it doesn't BURN any noticeable amount. I'll tolerate a good deal of leaking oil, so long as none of that is into the coolant, or past some piston ring or the rear main seal. Thus far, as I approach our 5th anniversary of C body Mopar ownership this spring, we have MUCH to thank the Good Lord for with this engine and the car it powers.
I likely will be selling the 906 and 452 heads I've got with the Commando and 400 engines, after I make a successful engine start w the 400. I was told the springs and rocker arms on the 906 were fairly new, and I'm apt to test them for this. If so, I may well be able to fit those 915s out MOST economically, IF!
I may even build that Commando 383 back up, though more as a low rev, low end optimal torque engine. Time will tell.
*vacuum hovers between 17 and 18 inches at that 500 rpm idle.