A work around of course is to use your tt "pitch" or fine speed control with a strobe to compensate. Stretching is definitely not the answer. If fact, a good argument can be made that a stretchy material is not appropriate for a tt drive belt.
Suggestions for stretching turntable belts?
I have had a great deal of dificulty finding TRUE 35.1" turntable belts (as used by Sota,the Luxman PD 300,B+O 4002/4004,the Empire tables and some older Pioneers.I have gotten them from many sources- PRB,Ebay,etc. and they all seem to measure about 34".Needless to say with a floating subchassis table this will not work as it pulls the platter toward the motor spindle.In some cases it will even jump off!Is there a good technique to uniformly stretch these? I have even painstakingly cut the belt in half length wise with an exacto knife to try to reduce the tension.