340 Running super weird

Well then its actually a Chrysler distributor with a FBO plate. If its a MP distributor the original curve was pretty good but the timing at idle rpms may need tweaking for a hot cam. Based on the information now, such as not using vacuum advance, I don't think timing is the issues your describing.

If you're interested in the timing advance curves, see my links in this thread Having fun with timing All the links are small block examples (even though the OP is working with a 383) with Chrysler advance mechanism.

If its not an DC or MP distributor, then its likely the initial advance is long and fast. The FBO plate can't correct that because it limits the advance at higher rpm instead of addressing the low rpm portion. But without vacuum advance it should be OK even with the limiter plate. And if you do everything the way Don recommends and use his springs it should work out in the end - at least I would think so - I haven't proofed it myself.