Here is my collections of pulleys, that I have collected in anticipation of adding air conditioning.
The top row of pulleys are ones I have gathered to add a 3rd grove to the harmonic balancer.
3376872 CH - I have seen this listed as being the correct one for 250 with air but it won't work with my truck because it is a bigger diameter than the balancer on my truck.
3790337 E I think this is the one I'm going to use...either that or the next one
3883263 AX This one has an offset to it. Like it would recess into the balancer(much like the 3376762 CH above)
3755820 BC - I'm pretty sure this one could work also.
The bottom row are 3 groove water pump pulleys
3906657 DP has two larger diameter grooves, much like my factory 2 groove. It has a 3rd smaller diameter groove that is smaller than the grooves in the next one...
326304 AI 3 groove all the same diameter
Here's another option:
http://www.vintageair.com/instructions/15249-vcb%20.pdf
They show some completely different ways than factory, but they have part numbers listed.