For Ultimate Ascent we attached a lead screw to a Versaplanetary with the 1/2" socket from this set (since we had 1/2" output shaft VPs)
http://www.harborfreight.com/7-pc-38...set-67929.html because we needed the swivel aspect.
We used a nominal 3/8" lead screw drilled and tapped it to put a bolt through the pin retainer hole and then drilled through the socket and VP output shaft for a cotter pin to retain the socket to the output shaft. We did choose to turn the end of the VP output shaft so that it would extend into the round hole in the socket because we did not feel the shallow depth of that socket didn't have sufficient room for the cotter pin.
If you do not need a swivel head to NAPA and get a single 1/2" impact socket in either 3/8" or 1/2" drive depending on the lead screw you are using.
As others have mentioned you can use a VP as a 1 to 1 but I think you'll find that a little fast. But you can always try it and if it is deemed too fast put a reduction stage in.