As I have been told, top chopping was originally done for aerodynamics on dry lakes race cars; the street cars simply picked it up to emulate the racers' style.
Any car can be chopped, but not every car should be. If you lay the windshield back on the '40 by pie-cutting the A pillars, IMO there should be no need for a second cab to stretch the top....keeping the windshield at the stock angle, on the other hand, would require a stretch when chopping the top. You oughta be able to Google somebody's in-progress or tutorial on chopping a similar style of truck in 1:1 as well.