This could be done quite easily with a welder at your service.... The rear end and suspension for a 70's Vega would work very well underneath this car... The rear wheelbase is very close, and the 4 link suspension should match up well. All you would need to do is remove the entire rear suspension from the vega, and weld it into place under your brat. The tranny crossmember could come too, but I'd recommend using a Turbo 200 or 350 (instead of that HUGE TH400) the 350 should fit well in the trans tube. You would have to have the driveshaft custom cut, but that's not that expensive.... And as for mounting the engine, that's nothing a little ingenuity wouldn't fix (Weld V8 Vega/Monza frameplates in, and lengthen the motor mounts) These cars are close in size to the Chevy H-bodies, and I have done the V8 swap to those cars on several occasions. I think a V8 powered brat would be awesome, and maybe only 2-3 weeks of work, if you planned it out well and had all the measurements handy!
Just my .02
Adam