Stock/standard size tires to drive in sand for '85 Marathon gas

[Shugs]

I posted the other day asking for suggestions to handle the additional 350 pounds or so I'll be adding to my '85 Marathon gas cart by turning it into a steel dragon art car for Burning Man.

In the end, I've decided to just get/install some new tires and new front/rear hd leaf springs (I just posted a question about the springs separately).

Given that I'm not going to lift the cart, and that I can only drive about 5 mph out there, I just want to get some 6-ply (or 4-ply?) standard/stock size tires that are OK in sand.

So, first question is what *is* the standard/stock tire size? Seems like all of the posts on this site are about new *bigger* tires (e.g., 22", etc.)!

Second question, does anyone have a specific make/model recommendation for the kind of tire I'm looking for? Also, call me lazy (and dumb!), in case it matters I think I want to get the full tire/wheel combo (vs using a tire mounting tool to get just the new tires on there).

Thanks,
Seth