Floorpan questions?

[1973mach1]

I was wondering if anyone could help me with a few questions about installing floorpans? First of, if using a stick welder and im tack welding do you drill holes in the pan were your putting the tacks? Im not doing the welding but my cousin is, he keeps burning holes in my floor though. Another thing is the floor pans dont match up exactly with the ribbing in the stock floors, in some spots the floor pans have a "channel" and the pans do not, how do I fill this space?

Thanks,
Jim