Hi Guys:
The alternator (70 - 75 amp) from a '90 - '94 Spider will bolt on to your earlier cars. I've used that unit on both my '74 and '87 Spider's without trouble. Of course the biggest improvement came on the '74, as it originally had a 35 amp unit. Now I can use the H4 high beams, defroster fan, wipers, and the stereo all at once. I had one tiny problem on both cars, though, which was easy to remedy. The fan on the new alternator (at least on both of mine) was ever-so-slightly larger than the original, so it rubbed (barely) on the washer under the head of the main mounting bolt, and also slightly on the bottom edge of the adjustment bracket. A little grinding of the washer and the bracket fixed things right up.
Besides the extra current output, the other difference is that the new alternator has its voltage regulator built-in. For the new alternator wiring, first remove the old regulator and the harness that connected it to the old alternator. Keep them or trash them, but they don't need to be on the car any longer. There will be one wire (green) left unattached in the car's harness, which is for the alternator light. Run a green 14 ga. jumper from it to terminal 'D+' on the new alternator. The big red wire from the B+ on the old alternator is the same on the new, but I replaced it with a larger 8 ga. wire, as I though the originla was undersized, even with the smaller alternator.
Also, I should tell you that others have used GM/Delco, Ford, or Chrysler alternators of various current output, which are usually available for less than $50. Some minor machining is usually necessary, and I didn't know if that was an option for you. I prefer to stay with a Bosch, anyway, as it's 100% bolt-on, and it looks original.
Here's a source for the Bosch alternator:
http://autoexpress.safeshopper.com/196/cat196.htm?201
$83, including core charge, and a 3-year warranty. Don't forget to disconnect the ground wire from your battery before doing anything, and ask if you have any questions.
Regards,
Dean
Lutz, FL
'74 & '87 Spider Veloce's