Sorry but I have never seen a belt STRETCH. Granted they might just a bit, but typically they blow up if pulled that hard.
personally I would check the center to center on your primary and secondary.
Don't know the correct number off the top of my head but I would start their.
Also their is the bump stop between the back of the motor and the jack shaft bearing mount. make sure isn't broken off or missing the rubber.
Again I have put a lot of miles on that chassis and never had a belt "stretch". I have had my fair share of blown belts and clutch problems but....
Thunder