Nicest... manifold... evar.
I've got a mandrel bender too at my uncle's old shop. Too bad I can't use it for shit. and I don't know to weld either..... time to starting learning :)
with enough thinking and plotting you could most definitely get the 4G mani to work with the 420A flange, just need to check the ports out a bit... and the picture makes them look relatively the same so I don't think it is that big of an issue.
another idea, if the mani hits things, machine a larger adapter, that's not just a plate, more like a block. probably not the most mechanically sound idea (bolting the mani to the adaptor to the engine) but you could accomplish two things ... better matching of the ports, and you could move the manifold out more, rotate it, angle it, however so to get the manifold right where you want it. it'd be a very expensive piece to machine though... making the entire venture basically worthless.