do you want to cut the top off because the car is a rusty POS and needs a roof or because it's just an old beater?
if it's past the point of no return, and you don't particularly care about the quality of the finished product then i don't see why not. but know going into it that you're looking at more than just a saturday afternoon with the sawzall.
as mentioned above, the amount of structural reinforcement is fairly involved. and i would 1000% add a roll bar.