Squareback vs round back alternator?

A bit late chiming in. First verify that you require the ballast resistor. Many people bypass it when upgrading the ignition. I didn't see where you opened the distributor cap to verify there are points. Someone may have retrofit a Pertronix Ignitor electronic ignition under the cap. Those were sold even back in the 1990's. Another is the Crane Cams XR700 optical pickup wheel, but would then have an external box. If it has points ignition and you ran it without the ballast, so the ignition coil + got full 12 VDC all the time, the coil would soon get very hot (enough to smell its paint melting). It the car had been running fine, that suggests you don't have a points ignition. Also, whoever wired the relay was smart so unlikely they would have eliminated the ballast without good reason.

The relay is good to give full IGN power to underhood consumers. I put one in all my 1960's Mopars (via fuse/relay box from 1990's Jeep). The terminal numbers are standard German DIN seen on all Bosch-style relays (even today). 30 is usually power in, but sometimes input on 87 (normally the output). I see 85 commonly for coil-, but polarity doesn't matter unless there is an internal diode across the coil (86 & 85). Since IGN and IGN2 are jumpered together (no ballast), IGN2 works fine to activate the coil. Even with the ballast you added, it may still work since the IGN2 wire will run ~8 VDC (key in "run") which should still be enough for coil+ to activate the relay. Note: when I say "coil" in this paragraph I mean the coil inside the relay, not the "ignition coil".

Re alternator, as stated you can run either the original roundback or squareback, using either the older high-side switching (one brush grounded) or low-side switching (2 field wires to alternator, one being IGN output from relay). You can wire a squareback to serve as a roundback (just ground one brush, doesn't matter which one).

Re the voltages you measured in post 72. I suspect you don't have points in the distributor (pop off the cap and show a photo). That is because at "ignition coil +" you measure 13.8 VDC (engine running). If you had points (with proper dwell set), you should measure ~8 VDC (or less). That is the whole purpose of the ballast, to reduce the average voltage at the coil (gets full 12 VDC when cranking via IGN2 via key switch). Whatever is switching the coil on and off is more efficient than points, so likely an electronic ignition. You don't need to restrict its voltage (except if an old 1970's Mopar ECU). It only drops 0.6 V across the ballast (14.4 - 13.8). The readings at IGN are "about right". Should be 12.6 VDC with engine off if battery is fully charged and ~14.3 VDC with engine running if alternator and Vreg are working fine. I am confused by your "IGN 1 and IGN from Vreg". The factory drawings don't have an "IGN 1", just an "IGN" and "IGN2". Also, IGN flows to Vreg not from it. Vreg is a consumer of IGN power and also tries to control it (via the alternator output).