I have the HEI dist. on my 69 - 396 and there is plenty of clearence to the firewall on mine.
I replaced the original resistive (white cloth covered) wire inside the harness from the bulk head connector to the distributor + (Bat) terminal (was on the old coil + terminal) with #12 stranded wire with a spade type "push on" connector at the distributor end and re-wrapped the harness.
Another way is to just run a new wire that is hot when the ignition is in the "on" and also hot when in the "cranking" positons.
I also abandoned the yellow - bypass wire that ran from the same + terminal of the old coil to the starter solenoid as it is not needed with the HEI dist.
Some people think a much larger wire is necessary to supply power to the HEI distributor but I have never had any problems with the 12 gauge wire.
------------------
Dean Call
A.C.E.S. # 00235
N.C.O.A. # 4350
macc.chevelles.net