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Is there an "optimal" hole size to punch into the cross member flanges for plug welding to the floor? I know from past experience that if they are too small it is difficult to get sufficient penetration and if too big you may risk blowing through the bottom.

8 mm or 5/16" works for me.

Thanks, I'll give that a try...

Drill/ punch as far away from the edge, to minimize the risk of burning through, and losing that sharp edge detail. 6-8 mm here.

Yes, good advice. Too bad BMC didn't seem to worry about where those spot welds wound up. I had to drill out spot welds that were halfway off the edge of the flange in many places! That said, I would love to have a spot welder with electrodes long enough to tackle this job. Problem is it would probably need a supporting counter weight/spring in order to manage the weight. I can barely deal with the heft of my "little" spot welder that I used on the external flanges!

The hole punch's you can buy for this specific job are usually 5mm
5mm is okay just...... for welding thin sheet to thin sheet, but only just

If the lower sheet is thicker gauge i dont think a 5mm hole is big enough
8mm is the size of a spot weld remover drill bit

I find 8mm a little big and usually use a 6 or 7 mm drill for plug welding, even that extra 1mm on a six mm hole seems to make all the difference.

Wound up with a 1/4" (~6.3mm) punch since that seemed to be a good compromise after practice welding with a few different hole sizes on some scrap material. I used my Roper Whitney hand punch and was able to shift the holes away from the edge of the flange. Worked pretty well.