What wheelbase did you end up with?
FWIW, I built my buggy frame from 2x3x0.120", butt welded just like yours, I have no worries that it'll give me any problems when its done.
Granted, I would follow different rules on a stock frame that is work hardened, but I don't see it necessary on new rect tube. Always interesting how people think stock frames are strong.
Looks very nice