Are you using a DC only or AC + DC amp clamp? The much cheaper AC only clamps won't measure DC.
Are you clamping it around only one of the main battery cables? Clamping it around both cables will cancel out any reading.
As suggested above, it's not a good idea to run the alternator with the battery disconnected. It can cause serious damage to the alternator diodes.
It's a large black or grey box fitted under the bonnet (hood) similar to the picture below. If it's there, you can't miss it.
If it's been modified to the later internally regulated alternator, the external regulator would have been removed. You should be able to see wiring modifications where the external regulator was mounted.
Are you sure it's a 1976 car, and not a later one? Several members on here have been confused as to which year there Spiders are. The identification pages below should confirm which you have.
You may also get some clues from the diagram of the two different alternators for the up to, and post 1976 models from the parts catalogue.
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