[GTJIM]

Leave it as an automatic and spend the money upgrading to a larger Opel motor - a 2.0/2.2 or even 2.4 litre version.
I have a 1972 auto and it was plenty nippy around town even with the 1.9L motor with high compression pistons fitted.
Auto GTs are very few and far between because everyone wants to 'stick it' to them!
Ladies love them too - so much easier to drive.