Low Alternator output

1971 classic fitted with a 1992 EFi 3.9. When we got it the alternator output was low giving about 12.2V both at the battery and at its rear connector. It had a Ford label on it so I took it to be a second hand unit at best fitted by a previous owner. I put one on from another classic in a breakers. Barely 13V now and last year it went down to 12.2 again. I found I had a spare kicking about the garage which was the same as the Ford one but with no label, fitted it and originally got 13.2V. Now today after a run of about 50 miles the output is again down to 12.2-12.6V both when checking at the battery and at its connections. Any battery that goes on dwindles to about 12.5V at rest and needs a jump to get it going. Am I just being unlucky with second hand alternators or is there a separate regulator on the car somewhere that isn't working as it should? Being an old 1971 car there is not very much in the electronic department on this car even though it has been given a 1992 loom and ECU. If fitting a new alternator would a 45/ 65A unit be best. A 100 or 150 would be a bit much seeing there aren't many electrics on the car at all. (Doesn't even have a heated rear window!!, just blower, lights, wipers and EFi.)

why dont yu get it checked by an auto sparks?

Think that's next actually. I'll take the two old units in as well to have them run up.

Duff battery.

.............giving a false reading, implying it's charged when it isn't? Would that trick the alternator into thinking it needs to stabilise at 12.6, suppose it would - thats me thinking out loud. Known good battery going in today methinks.

Decided to check the earth connections. They were secure at all points however they were corroded and gunged up. A quick rub with emery and a re-tap of the connection to the block and bingo - 14.2V at the alternator and starter spinning like a Mini. Lights are brighter and it's running a bit smoother now (at last). Yes, I know that is tempting fate and the fate monster has bitten - just spent a day getting the clutch back to normal after trying to bleed it when I noticed the fluid was low. Fixed now though.