To add to Dave's, tubing is more rigid in span than solid, and there're lots of configurations of "flat" tubing. Are you planning to atatch your new "crossmembers" to the frame or floor. If the existing floor is your choice, and in good condition, then plug welding the new cross pieces will spread the load, and you can get away with a smaller cross section tubing/channel. If attaching to the frame, then you need more rigidity.

Since you're talking about a pad sounds like you're attaching to the floor. The typical factory pad from that era was similar to tar paper, about 1/4" thick, often with a waffle pattern. Here's something similar offered by CPP; http://www.classicperform.com/Store/...s/4972GTMP.htm
Although, if noise insulation is the main objective (and a good one) a piece of aluminum foil/bubble wrap type insulation would do the job, cost next to nothing, is available at your local home supply store and won't hold moistrure.