Generally, vacuum on the street, mechanical on the strip. However, I have run mechanical on the street without overheating or other problems. My '68 GTO with a very radical re-build ran a Unilite, as did my big-block A-bone. I had both worked over with a wizard on a distributor machine, though.

I'm running an HEI on my '34 ford w ZZ-430 clone.

I'd say if you have a mechanical, give it a try. Just make sure you have the right curve built in.