i'm using 2x3 1/4" wall on mine. should be getting started in the spring. gonna build the 383 i bought to go with the frame first. i'm shooting for 500 at the crank + 100-150 shot of nitrous.
650 horse And 2x3 tubing.....cmon, 3x3 3/16 is way stiffer and stronger....plus if you ever want to tow, or put something in your truck...SLightly overbuilt now will be better in the longrun.