perhaps air metering and control? IAC, IAT, MAF, MAP, TPS, or O2 (long shot, addmitedly). If you're on Discord, ZZP has a pretty robust server going. Let me know if you want those instructions. Your issue doesn't ring a bell as to something we've faced here. My first thoughts went to a fueling problem, as well. Since you've ruled that out, I would go to breathing or spark. I'm going to assume you've already replaced the plugs and wires with your rebuild. Testing the MAF is pretty easy: with it running, you should be able to unplug it and it runs fine. When you unplug the MAF, it should start to stumble and roughen the idle.
_________________________
WizAire CAI w/9" K&N & HV3, 2.5" d/p w/h/f cat, Cobra CB, PowerSlot slots w/Hawk HPS, GR-2s, GMPP handling kit, NextLevel STBs, Pilot Sport A/S Plus tires.
98 L36/MM5
RaC