I would check in order...
Fuel filter getting clogged
Fuel cap not venting
Bad HEI module
Poor engine ground wires to chassis and engine
Faulty fuel pump
When you put the HEI in, did you remove the resistance wire power feed and replace it with standard wire? The HEI needs full 12 volts to the module. The resistance wire used on old points systems drops this down to 9 volts to slow the burn up of the points.
