CK, I am in the planning stages of building a drilling machine. The client wants a machine that can drill dia 20mm holes in a dia 50mm stainless steel pipe. Some of the pipes are a couple of meters long. I'm thinking in the same line as you to make a frame from I-beam (about 4" x 2") and then make a bridge where i can mount the pillar of the drill.
The only big obstacle is to get the two rails (beams) 100% parrallel.....
Klox