Since it sounds as if the HEI was previously installed, how did it run before? It sounds like a ground issue or voltage to the HEI itself is a problem.
When you say you have voltage @ the distributor but not after, exactly how much voltage do you have (for sure)? Run a hot wire (10guage) from an ign-unfused source on the fuse-block directly to the batt terminal on the distributor & see what happens. If the rotor is turning (no broken timing chain) you should have spark. No spark, look @ the coil & it's ground strap.
Keep us posted. We all love a good mystery......
we just hate it when it happens to one of us & in this way! 