Hole size for floor pan spot welds...

I always go with 5/16". Just make sure to keep the heat focus on the thicker base metal.

Why 22 gauge? That seems awfully thin. Do these old Mopars even have any pieces of steel on them that thin?
I read on a few threads here that the pans were originally 19ga. The pans I've been looking at, I didn't see where tgey said the guage of the steel. Anyway it was a simple choice of I just went to Lowes and they had 22 and the next thickest was 16. Thought using the thinner material would be better practice to ensure I'm also not burning thru.