I dunno if it would be street legal since your be modifying the safety of the car, if you rear end someone with a tublar frame im sure your smash the **** outta them, nice lawsuit on your hands.
My dads car has a fiberglass front end and tubular crap, and drag shocks, not the nicest thing at all to drive on the highway
But can it be done? Of course, anything can be done if you got the money.
They also have tubular K members that save on weight. You could also get fiberglass hood, trunk, and doors and save alot of weight.
But sounds like you want a race car more than anything