Mine seems to drop spark at about 4,000, the timing light gets erratic. Trouble with the gm module is there are quite a few different part numbers and each one has different characteristics such as dwell. The first one I tried this time around was from a smog 455, 73 I think, I put in another and got better results. Can't seem to find a chart showing what each different module does. I also set the initial to 16 degrees, ran it up a couple of times and had an initial of 24? The distributor is tight and the advance is working.

Is your ebay procomp an HEI? If so do you by-pass the module with the MSD box? Can the ignition box be used to modify the advance or is it just to give a consistant hot spark?
I looked at MSD distributors with the adjustable timing stops but The largest stop bushing will only limit the mechanical to 16, I need to limit mine to 12.

That in cap coil gets pretty hot, especially with a modified engine and tubular headers. The one I took out of the HEI when I bought my car really showed signs of heat damage. I'm surprised it worked at all, most likely wouldn't for long..