The only draw back I see with the MR2 is
1)Price, they can be super expensive for one in great shape. Try to find one that needs body work or paint.
2)The handbrake is located in the wrong area. Unless you can figure out a way to relocate it, the build will be compromised a little.
Now as far as compensating for a larger drive train, that is up for debate because I've seen guys swap in V-12's into Fiero based kits before where as most of the MR2 LS1's I have seen barley can fit.
I vote MR2 because its easier and quicker to stretch vs the Fiero where the chassis needs to be cut by the fire wall along with the engine create, the suspension needs to be widen, it's just a lot more work on the Fiero. The MR2 seems more ready and complete
Really? how many MR2's or Fiero's have you stretched? I know from experience the MR2 is more involved than a Fiero.
So, you dont need to widen the MR2 suspension? You have to cut the engine cradle in the Fiero? I dont know of one person who has cut their cradle to do a stretch on a Fiero. If they have, they obviously dont know how to stretch a Fiero.
You have to cut the entire tub/pan on an MR2, then fill it in and reweld everything, far more welding and reinforcing than whats req'd on a Fiero.
If Mitch stripped and stretched his MR2 in 7 hours, he probably could have done a Fiero in 5. After the MR2 is stretched, the rear firewall is in the engine compartment, well behind where it needs to be for a G28 build. The stock dash sits way to high, and into the side windows past the mirrors.
The amount of reinforcing that needs to be done to a Fiero chassis is far less than whats required for and MR2. Ask somebody who's done both (not just stretching the chassis, but mounted the body and done the build).