First of all, you don't have to use the 4.3 TB, as long as all of the sensors on the V8 TB will plug in...there's no difference in the TB itself, only the injectors...as someone pointed out above. On my 350, I put the 5.7 injectors on the 4.3 TB so that all the sensors would hook up. I'm currently building a 383 to replace the 350, and since I have access to the CNC machine shop, I bored out the manifold to accept the Holley 670cfm TB that I just scored off craigslist...which is the same size as the stock 454 TB, except the Holley unit came with an adjustable fuel pressure regulator. I don't think you'd see much of an increase in power from upgrading to the larger TB, if you're still putting it on a stock manifold. The manifold itself would still be a restriction. So you could try the adjustable regulator and see how much power you can get from that before you run out of fuel flow from the pump. The guy that sold me the Holley TB said that the stock pump was making the 18psi that he had the regulator cranked up to, but at that pressure it ran out of volume...so he said I'd have to upgrade to a higher volume pump. If you really want to upgrade to the larger (Holley or stock 454) throttle body, I'd recommend swapping to a square-bore carbureted manifold, and then buying the right TBI-to-square-bore adapter plate. Or if you know a machinist... you could trade, barter, or pay him to bore out your stock intake. The reason I went this route, other than having access to the machines and wanting to save some money...was to gain flow through the intake, while still having all of the sensors and accessory brackets bolt right up to the stock intake, and not causing any other issues there. I might consider boring out stock intakes for other people, but it still wouldn't be super cheap...considering the shipping costs, and the time each one would take.
By the way, from what I've found on the internet, I'm pretty sure that the stock 4.3, 5.0, and 5.7 TB's flow around 500cfm, and the big block and Holley units are about identical at 670cfm. I'm personally really curious as to whether a set of stock 454 injectors would bolt right onto one of the smaller throttle bodies, or not...
Wow...I really wasn't expecting to write that much...but hopefully somebody finds it helpful...
CK
1987 Astro RWD
TBI 350/700R4