It all depends on what he wants out of his 68 Camaro. The biggest difference between a TH400 and a 700R4, is the fact that the 700R4 is an overdrive. The 3rd and final gear in a TH350 & a TH400 is a 1:1 gear ratio. That's the same gear ratio as 3rd gear in the 700R4; but here's the difference, 4th gear is a .70:1 ratio. In short, his RPM's will drop by 30% when he shifts into 4th with the 700R4.
However, there are drawbacks. 1) The 700R4 is weaker than a TH400, so if plans to do a lot of drag racing, odds are, he's going to break it. 2) The cross member and the driveshaft are different, but that is virtually simple things to replace. 3) The shift pattern will be different. Another words, if he's running a factory floor shift car, the shifting mechanism was only designed for a 3 speed, not a 4 speed. When he manually shifts it to the lowest gear, the transmission will only be in second, not first. Hope that makes sense.
As for the stall converter, that depends entirely on your engine, and the camshaft that is in the engine. A stall is only good, if it's needed, otherwise, you're throwing your money away.
Hope any of that helps you.....