Which Chevy fenderwell headers suit (early) A-body best?

Yes, the big block ports are spaced wider than the small block ports. The gasket overlay suggestion above is a good way to see the difference. IMO, the reason why these may be more suitable than chevy headers is they are designed for the mopar chassis and unlike an under chassis header, fenderwell headers will have much straighter tubes coming out of the ports and have a longer tube sections before a bend to allow them to be pulled into the small block port layout. However, they also are going to be much larger diameter tubes than may be desired.

In either case, big block mopar or small block chevy, I doubt this is as simple as cut the flange off and reweld a new one. There may be additional nip and cut and welding required to get all the tubes to clear everything. But either one would be easier than making a custom set from scratch, at least for us average enthusiasts.