RE: No foam inserts?
While crawling and your tires hold air you will bounce like crazy, not a very controlled way of crawling. And your tires will seem harder on hot days and softer on cold days as the air trapped inside will expand and contract with Ambient Temps. I would remove the tires drill 2 1/8" breather holes in the rims and reinsert foams and find a "cut" on the foams that works for you. Or you could get some memory foams, there are more and more memory foams becoming available, I like prolines since they are inexpensive and seem to work well for me, but you will have to experiment to see what works for you.