Don't know what's causing your acceleration problem, but these cars do have a history of charging problems. In my experience, rebuilt alternators are all over the map, and I have been through at least a half dozen or more for the four 5.0 cars I have owned. Some have seemed to charge fine at idle, while others just have low output that heads dangerously red when idling with lights and accessories on. Personally, I've sworn off Pep Boys rebuilts, especially now that they've rescinded their lifetime warrantee. I guess they got sick of replacing their own crappy alternators multiple times. I may seek out a custom high output alternator next time around (and I'm pretty confident that it will happen sooner or later). I also suspect that wiring plays a part too. I currently have 2 '90 5.0's, and one has consistently low charging characteristics, even when I swap alternators across. One day I will try to hunt down bad grounds and connections if I ever get around to it. But to date, I can't pinpoint any one likely cause for the low charging. In any case, 8 volts at idle sounds low.