I think the OP has what he needs here. Shears to cut up 18g steel and his replacement pans will be 20g.
I've got both pieces in my garage at the moment and the OE pans do not measure up to 18g but I agree they are somewhat "thick" to be 20g. The aftermarket floors pans are definitely 20g steel.
I've been welding with a Millermatic 220 and it seems to work well just using the 20g auto set.