Rough idle - wants to die - only on hills?

[Gibson]

My '78 Silverado just got back from the tranny shop (rebuild and shift kit) and I noticed it was acting up on the way there (running rough and sputtering a bit). So I had them check the timing and give it a tune up. They said it had been way retarded, and advanced it to smooth it out @2500 RPM.

Nice guys, and I trust them. I'm sure it was running great there at the shop. But on the way home, I went up a hill and it started acting up again. My driveway is a hill as well, and when I let it idle in the driveway it starts to get really rough and want to die.

I have not tested it again on level ground in case this was a sign of something more severe.

Could this be fuel pump? Or am I imagining the hill thing and this sound like timing? Is there anything that would make it run this way that you guys can think of?

So frustrating! All I'm wanting to do is