do a search in the archives - there are a few methods for stretching. one that comes to mind is: with the top up, in the hot sun, "lean" on the top between the bows with your arms. do this on both sides of the car, in front of and behind the top bows. leave the top up as long as you can, the canvas will eventually stretch.....
Bob Happe
Pittsburgh, Pa
1970 280SL, white/black 4 speed