I'm an autocrosser so I'm out numbered here but I'll throw this out anyway.. would an LE5 from the solstice/sky be a decent fit. It makes more power than the base v6 and not much less than the v8 (stock for stock) plus being an all aluminum engine it probably weighs a fair bit less too.

 

Depends upon what you mean by "decent", and what you mean by "fit". I'm sure it will require a good bit of fabrication, and getting the engine management system in the 3rd gen will surely be a challenge. The turbo version would surely have the better power, but I'm not sure how well the peaky power would do in autocross.

I'm not all that familiar with autocross classes, but I'm sure it would put you in a highly modified category. I would think an LS1/T56 would be a better choice.