Duplicolor sells a great undercoat in a spray can that is very good for doing what you are looking for. About 4 cans will do your floor & frame = 1 coat is all you need & it does not add alot of weight to the rig.

I just did my 71 chevy frame inside & out followed up with a semi gloss krylon black to keep it from fading.

Or you could use the roll on bedliner material for even a tougher product. Sold @ Wally World or Pep Boyz.

The cost is 1/2 if ya use the spray on.

Why go around? When You can go over!