Well, it's kinda time to go back to ground zero and start checking things.
You say you changed cams?
1. Are the timing gears set up correctly?
2. Are the valves too tight? Loosing compression?
3. Is the dist. installed with the rotor pointing to #1 wire when the engine is at TDC on #1 cylinder?
4. do a compression test. ALL low would make me re-visit the cam/chain install. Some high and some low would tell me to re-ck the valve adjustment.
5. Did you loose the ground to the HEI? Crappy spark will cause a timing light to not work, or work sometimes.
6. Is the HEI a known good unit?
7. Was the engine a known good unit? LOL
Just things to check. Sometimes it's good to go back to square one and see what got missed.