Years ago thought I could find a smaller sized alternator, such as, Denso style to fit into the GT. Just couldn't find what I wanted.
If you have to replace alternator, I had excellent results with 13050 (45-50 Amp) from
Online Auto Parts and Auto Accessories Store at PartsAmerica.com instead of 13009.
Look up 1975 Opel instead of 1969 to 1973 GT. Many Owners have switched to GM style alternators also.