If by overall performance you mean acceleration, go SBC. Lots and lots of info on the net, been done thousands of times. Google V8Archie. He makes kits including everything needed for the swap.
If by overall performance you also mean handling, the V6 with a turbo or SC is the way to go. SBC ends up being too much weight towards the back IMO. I also believe the V6 requires less modifying and cutting.
You can build a SBC with 400-450hp pretty cheap. Power to weight ratio will be insane.