Hi,
Instead of doing this yourself, you may want to consider contacting a race car, kit car or performance go-cart or off road vehicle manufacturer. It will probably be cheaper for you in the long run. You could figure out what you need for bend radii and weld the bends to straights as another option.
If you want to do it yourself, then the comments about filling the tube with compacted fine sane or cerrobend. You will then need a form or mandrel that goes about halfway around the pipe to prevent kinking of the tubing. Mention was made about heating the tube. Filling the tube would still be required and make sure you don't quench the tubing in water, only the wood form! A bucket quench will make the tubing brittle and that is the last thing you want for the frame.
I re-purposed a Harbor Freight pipe bender to bend tubing for part of a sidecar frame I built. I made a sheet metal shoe to go between the mandrel and the tubing being bent to compensate for pipe to tube OD dimensions. I've attached a couple of pics showing this. I filled with fine "play sand" I got at Home depot or Lowes. the tube had an 1/8" thick wall.
Thicker wall tubing will be less likely to kink than thin wall but requires more force to bend. An electric conduit bender will work on only thin wall. When you bend, if using longitudinal seam welded pipe or tube, make sure the weld is on the neutral axis like my picture, i.e. in the middle of the bend when looked at from the side. The tube below this line is in compression and above in tension. Bending always tries to go oval or kink the pipe or tube and that is why you need filler on the inside and a mandrel on the outside.