Built the tail jig, using kiln-dried Douglas Fir 4x4s for the uprights and crosspiece. The 4x4s were hand-picked and were the straightest ones I could locate in Rockford, IL. The vertical pieces were bolted to the floor, and I added end braces to make the jig very stable.
After it was in place, I established a centerline on the jig, and installed overhead plumb wire that passes thru the post, then runs to a tension turnbuckle on the side.