Hey guys, I've got (4) 3/4" holes to cut in a truck crane mounting plate that is a piece of 1" plate welded to a piece of 3/8" plate. After 25 minutes of heating, I got the top plate hot enough to cut through the 1", but as soon as it hits the 3/8", it stops cold.

I'm going to try heating the 3/8 plate from the underside next, but I'm out of acetylene, so I figured I'd solicit opinions while I'm stopped.

Any suggestions would be appreciated.
Thanks!