Hei swap on my 1972 eldorado convertible

Instead of the HEI you could install a Pertronix Ignitor-II and ACCEL cap and rotor in your original unit and keep your ability to run better coils and wiring.

HEI is really no more or less than the Hall trigger setup. The Pertronix is probably 1/3 the price. We use all Pertronix in our Olds 455 boat distributors.....converted stock old points units with never a mod failure.

I threw in the used HEI today-had to tweak the hard breather/vent copper line and remove the inner AC compressor belt as expected and it dropped right in. I also had to modify the plug wire end with some old make-it-yourself-crimp on HEI ends I already had from an old kit. One 12 volt 10 gauge wire to the cap/coil and she is cookin' for a 20 bill. If you use the old coil wire-make sure it does not have an inline resistor pack...the HEI needs a full 12 volts and plenty of amps.

I used a distributor from a 1975 500CI engine

Good move. The HEI ignition is the best thing that has happened to older GM cars with points. I've done that to Chevies, Pontiacs, Olds, and Caddys.
Of course the Pertronics are a good way to go also, like submariner sez.