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There are aftermarket HEI distributors that use a remote coil that are the same size as your current distributor. It is only the coil in cap HEI's that are big and cumbersome and could though not necessarily will interfere with your fire wall). Problem you will encounter with an aftermarket HEI is the Red or Blue or Beige distributor caps they have (though some also offer a distributor cap in basic, but black not all of the companies do). Of course this is only a problem if you wish the motor to look stone stock. Of course stone stock is a open to debate (unless you are talking chalk mark original). This is because originally only your mom had a stock car, everyone else modified theirs to make it their own car.
Back in the day I ran either a Mallory four lobe dual points distributor (still have one hanging on the wall in the tool room), or a Vertex magneto (which also used points to fire the ignition) because stock points started to bounce at high RPM and the coil couldn't keep up with the firing demand (the reason we have coil on plug now, and I had twin coils fired by two sets of points when I wasn't running the mag back in the sixties).
One of the coolest looks for your muscle car in the late sixties along with the Clay Smith Cam and Dean Moon's Moon Eyes decals was an Accel distributor and an Accel "Super Coil" firing your brand of plugs (back then that would be good for an hour long discussion as to which plug brand was better) by way of bright yellow and red tipped Accel spark plug wires. This was because MSD had not been created yet, and Mallory didn't sell anything else other than a great high performance distributor.
Accel makes a HEI small cap distributor (it uses a remote coil, and yes Accel still make their Super Coil); but to add to the small cap mix is MSD, Petronix, and DUI, as well as a boat load of Chinese knock offs.