do not believe there will be enough room for an HEI large cap (coil and distributor in one) to fit.
The external coil and small cap factory HEI was used on a many pick-up trucks and early Camaro's. Be carefully of the year of the donor vehicle; as an external cap HEI off of an OBD computer controlled car won't work (you can count the pins on the ignition module to determine if it will work. Four pins is what you want; five or more won't fly).
MSD, Accel as well as Mallory all make a small cap HEI unit that is better than the factory product because of improvements to the distributor body (using roller bearings instead of brass sleeves, and a billet body instead of a cast unit). The major difference is found in the ignition module itself. The aftermarket unit uses different components that shed heat faster than the stock unit can to maintain a steady spark above 4,500 RPM where a stock HEI drops off in spark energy (the stock factory coils refresh rate also enters into this, which is why the aftermarket sells coils matched to their ignition modules).
Finally if you want it to look stock you can rework your old distributor by replacing the points with a self contained Petronix HEI ignition module and slip a reluctor ring over the distributor cam. There are people on the web who advertise this rebuild as a service (worth considering if your distributor needs to be rebuilt to compensate for years of wear).
I installed a ZZ4 with the stock HEI in my 72 Nova. At the time I did some research and found that it was about 50/50 that the firewall might have to be altered to clear it. Mine fit with no problem but a tip from some was to replace the mounts on the front subframe, which might give back some space lost with aging originals. There were also the "make room with a big hammer" suggestions. I don't know how much difference in spec there is between the 66/67 and the 68/74. It might be worth a trial fit before altering anything.
As an aside, you will have to replace the factory resistance wire to the coil. The HEI requires the full 12 VDC for start and run.
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66 Beaumont Sport Deluxe, 72 Nova SS, 2009 Solstice GXP Coupe (wife's toy)
There were also the "make room with a big hammer" suggestions.
That's a popular suggestion by some.
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Prince Edward Island
'64 Parisienne CS "barn find" - last on the road in '86 ... Owner Protection Plan booklet, original paint, original near-mint aqua interior, original aqua GM floor mats, original 283, factory posi, and original rust.
I put an MSD with a box on my 70 Acadian. The firewall had been bashed for an HEI already. I found with just the MSD and the old box, that the engine fired up easier and quicker.