Why would you?  There are plenty of factory cabs out there from around
that time.  Plus a factory convertible, in addition to having the roof
removed has reinforcement of the frame under the car to make up for the
loss of rigidity provided by that roof.

All-in-all, a project without much upside and a lot of expense.
Now if you wanted to chop a 4 door, that might be a bit more interesting...

-Fred W