Never add a high power - high amp alternator without also upgrading the front engine wiring harness, and doing the alternator field hookup - interior voltage regulator bypass re-wiring--- i.e. adding a heavy gauge wire directly from the alternator output to the starter relay and/or the battery positive terminal. Adding relays is also great advice.
The wiring on your car is not up to modern standards, is likely old, likely brittle and would require an upgrade to handle a 90 amp alternator. When these cars were new in 1968 - 71, the most heavy duty alternator available from the factory was about 60 amps. Moving to a 90 amp alternator is not advisable and will be beyond the wiring's limitations.
In summary upgrade the wiring harnesses first before you go to a high powered alternator.