Thanks so much for the replies! Yes, I do mean the seat tube, not seat post. For every ones information- I'm welding the bottom bracket shell of the top frame to the seat tube of the bottom frame. I got a 1'' piece of pipe and stuck it down the fork tube through the head tube of the bottom frame and welded it at the bottom and at the threads. I then recut the threads to get the head set cups, spacers and nuts on. I cut off each arm of the forks on the top frame, leaving just the stump. I'm now cutting the pipe the the correct length and welding the stump to the pipe. Using 2x4's with c-clamps on each bottom bracket shell to keep frames straight and centered. The welds so far seem pretty strong and solid. didn't know if they were prone to crack anymore than any other welds or tubing in general. Definitely will be putting gusstes in front of and behind the bottom bracket shell on th top frame and seat tube/seat stays on bottom frame. Thanks again for the replies!