any sugestions i see the mini truck guys using 1.5 dom tubing and 2x3/2x4 dom tubing

think the 2x4 would be strong enough for the main section and tube the back half with 1.5 or 2inch

 

for the square tubing I would run 2x4 minimum, you can use .120 wall but strength all depends on how you go about bracing the frame with cross members and bend designs. This going to be a 3/4 frame or full frame?