"I wouldn't trust a pop rivited floor pan to hold my seat"

I would not either... and of course the steel to steel rule should be followed for several reasons...

typically one would use the air tool which will cut nice holes fast along the edge of the piece then turn that tool over and use it to flange the larger metal so the edge would be flush.... then the piece would be placed where you wanted it... and 1/8th inch holes drilled...and pop rivets used to hold it in place... or Clecos if you did not want to leave any rivets...... then at the places where the holes were made along the edge you would plug weld.
I believe I have posted pictures of that tool.