The dirty secret is that unless you can take an alternator to a shop that will load test at various rpms, or you do the same on the engine, no way to really no what you have. When these things get exchanged for rebuild, some places did and do 'em in batches and mix the parts up. New generic parts often get used as replacements. If what you have on your car is working, call it good.
If the only electric modification to the car is the MSD, that's not going to add more than 1 or 2 amp load at normal street speeds.
The original alternators had matched rotor and stator windings. If they had a high draw rotor, they came with a heavy duty regulator. But now, its all a roulette game. The aftermarket 'new' has a lot of garbage too. They play all sorts of games with names and labelling and 'ratings'.
In theory, a first generation squareback generally should have better output at low rpm. It will mount almost as well as a roundback. However I have isolated field roundback I got a junkyard that I keep on the shelf as a backup. Its only at a stop light on dark rainy nights that you really notice the slightly lower output at idle.
