From the years 1966 to 1976, is there any particular year/model front disk brakes (big bolt pattern) that are considered better than the rest or are they pretty much all the same? I'm planning on converting my GTX to disks and was just curious if I should look for a specific year/model brake set-up to use for the conversion. I have a set of 1974 A-body disk assemblies as well as a pair of 1972 or 73 E-body assemblies that I could use but if there is a better choice I would just look for those. For those who might suggest buying aftermarket brakes, I am not interested as I would like to use original Mopar brake assemblies.

Well, your gonna have to measure the inner diameter of the spindle to determine wether you have a 72 or 73 spindle.
The 72 uses a smaller diameter inner bearing, and a 73 uses a larger inner bearing.
The 73-76 A body spindles (brakes) are pretty much considered the ones to find and use, as they have the large bearing.
If you do have a 73 E body spindle, (or 74) those are a good one, as they are the same as the A body.
Confused yet? lol

Best factory brakes are the 11.75 rotors. The larger rotors fit any '73 or newer ABE knuckle so you already have what you need. Just buy the 11.75 caliper adapters from DoctorDiff if you don't have them then get the 11.75 rotors at a local parts store.

That may be true, but a personal preference.
But you neglect to tell the op that if you go that route you need 15 inch wheels to clear everything.

The smaller rotors on those A-body disc brakes stop a B-body just fine, and the stock 14" wheels fit on them if you,want to use them.

What wheels do you run on your GTX?

for the A body spindles, the 73-75 calipers have a 2.60" bore and the '76 ones at your parts house have a 2.75 bore so ask for that year, more front bias is almost always better. the outside dimentions/fitment are identical on those (3) years for that app, just a bigger bore/piston. EDIT (4) years

I will have to measure the E-body brakes tomorrow to see what the diameter of the inner bearing is.

15" Cragars.

https://www.allpar.com/history/mopar/brakes/disc-main.html

Use the 73-76 A-Body Proportioning valve, get 7/8" bore rear wheel cylinders and use a 15/16" or 1" bore master cylinder.

Dr. Diff has all the parts is you cant source them locally.