I have seen and driven the Solstice with the Chevy alloy LS2 dropped in. 1. It fits without major mods. 2. It weighs I believe within 80 LBS of the stock turbo motor, and with 400 hp and 400 lb ft it goes like h*ll.
One of the forum members, (V8 Archie) who does the GT40 on a Fiero kit...