For the HEI I would look at DUI Daves unitfied ignitions. Not as inexpensvine as the ebay dist.
Also don't get one for the 74' it only has 7 degrees of advance.
MTS offers Electronic conversion kit for a cadillac point dist. Put one in my old correct craft ski boat work with no problems for years.
Through some trading I ended up with an MSD AL 6 box, blaster high vibration coil and dist for my Cadillac. The MSD ran great with no issues for over 5 years. I can not say it was any better or worse than a correctly setup HEI with a good coil.
If you anyone looking for a caddy motor send them my way.