Converting a hard top to t-tops is a more involved process than cutting the roof. If you wish to undertake the job, my suggestion would be to cut the complete top off a parts car that already has t-tops. Then use the complete assembly to convert your car.
I was wondering about that but i have read on other forums that if you are fitting Hurst tops it is easier to do it the same way as they did originally. I am looking at buying a 77 SE that is documented with hurst hatches but now has a solid roof, but the guy has the original roof section with the hatches still in it to use as a pattern.