I recently had board member 70shortfleet convert a 70's automatic van tilt to manual for my 66 K10 Suburban. He does outstanding work and can customize them any way you like. I cut the original manual steering shaft to desired length, then used a Borg steering coupler (welded to original steering shaft) to join the two together. I have found that in most cases, the spline on the van tilts is 3/4 in. 36-Spline.
https://www.summitracing.com/parts/BRG-313400