DON'T DO IT!!
Any aftermarket sunroof that you add WILL leak and probably end up looking like crap. The only sunroof I would consider getting would be a factory fitted item.
Reason - I work in the auto industry and know that vehicles optioned with a sunroof from factory have ALOT of extras added to accomodate the sunroof, ie drainage tubes, extra seals, specific steel roof ribbing (spaced for the sunroof cutout), specific headliners with the cutout etc.
You're also dealing with a vintage and model of car prone to roof rust aren't you...?
But at the end of the day it's your car and you could probably find a tonne of places that would be more than willing to take a plasma cutter (or angle grinder...) to the roof of your car.