I think if you cut a glass Speedster windshield you'd then have to fabricate a shorter frame and center tensioner (which would tend to put the frame right in the driver's field of vision). Either that, or devise and new and better way to anchor the windscreen to the scuttle.
Sounds easy and fun, no?
Another option would be to buy some lexan and shape it. A heat gun could theoretically work to do the whole thing but the way I see many of these on the real cars is there's a wide bend on the face of the windscreen, which stands almost vertical, and its base is cut to conform with the cowl, where it's riveted to an aluminum angle bit that is in-turn bolted to the car. As in:
So, also easy and fun!
Whereas, the no-fun way is to simply buy the available plexi short screen and screw it on.