So I finally got my new distributor, a Procomp Thunder series with clear cap and rotor. I installed my new distributor and fired the truck up but I just can't seem to get the timing right. If I set it 6 or 8 degrees left of 0, the truck will run but has no power and misses terribly. If I set the truck even higher, it stops starting due to kick-back. Anything else that I've tried results in missing and pinging. Is there some trick that I haven't tried? The old distributor was without doubt bad. Is it possible for a slipped timing chain (just brainstorming) or any other reason?

1966 Chevrolet C-10 LWB
1965 Chevrolet C-20 LWB