2"x2"x1/4" is way overkill. You're building a lightweight car not a truck! No more than 1''x14g tube is going to be needed and probably less than that.
Space frames are relatively easy to analyse for strength. I suggest looking at Costin and Phipps book for a worked example. The book is old but the principles haven't changed.
Pure strength is not enough for safety (if that is the aim). You need crush space to provide the distance needed to reduce an impact acceleration. The frame just has to react that acceleration.
Rather than mess with adaptor plates a single engine and transaxle would be easier. The Audi 1.8T and Subaru EJ20T are readily available and have good aftermarket support.
Why build a chassis at all? If you can live with a single seat an older formula racer might provide the basis for a road car. I know there was a guy in Japan who registered an F3000 chassis for road use there. I'm sure that it has been done many times. Ford (Europe) has even done it:
With at least lip service to fuel efficiency, that one is a bit of us ecomodders I think; 1 liter displacement plus boost, light weight (485kg).