All out of ideas

[84Longbed]

Aright where to start...
I have a 84 c10 (Scottsdale) 2wd. When I got it, the 305 and 700r had been ripped out and replaced with a 350/350 combo. Im unsure if the motor had been rebuilt, but I was told it had a mild cam in it, and it has a edelbrock performer intake. I threw Hooker headers on it a while back, and it ran a little better, but not much. About a week ago, the truck started stumbling whenever I encountered an incline, kind of like it was running out of gas. I replaced the plugs and distributor, and now instead of stumbling on hills, it does it at low RPMs, like when I accelerate after a turn. It comes and goes also. I tried to set the timing, but while watching the mark on the harmonic balancer, I could see it moving a little like it had slop in the timing chain. And while im on the subject, I think the motor may have the wrong timing tab on it as its a little towards the driver side of the balancer, but i got it to run best with the timing mark around the 12:00 position. How can I determine what year my engine is so I can make sure I have the correct timing tab, and is there another way to check for a stretched timing chain or if it has slipped a tooth other than watching it bounce around with the timing light? Thanks in advance for any help, I sure could use it.