Hi - I haven't been able to get tach working.
Bought a 72 heavy chevy about 3 mos ago. The engine was swapped
before I got it. Looks like a 80 350 pickup [block # v 03 09 TDR].
Just had engine rebuilt now it is running..
I don't know which wire to hook up to HEI dist that was at original
volt reg. Tryed the brown but wont start then.
HELP thanks

Is it a factory tach? The factory tach wire is brown and will exit the engine harness at the dist, is that the wire you tried?

Yes it is a factory tach. I tried the brown wire that was in the harness
that went to the original voltage regulator. The engine wont start then.

Wrong brown wire

The wire would have connected to the original coil not the voltage regulator. If you connect that to the HEI dist at the terminal labled tach. the engine wont run? If so it sounds like the brown wire or possibly the tach is shorted to ground.