Torque spec is "tight enough to seal without bending the exhaust flange". Retighten after the first run and they will stay tight thereafter.
Seriously, getting a torque wrench on the exhaust nuts would be pretty difficult, though not impossible with a long extension. I believe it's more important to get the two nuts evenly loaded so the exhaust gasket is seeing approximately even pressure all around.
I use loctite red 271 on the stud to head threads and none with the self locking jet nuts (because they really don't need any). In fact I use a tiny amount of anti seize on mine, and they do not loosen.
I should also note I replaced the bent stock exhaust flanges on my M3W with nicely machined (and straight) Harley units. If your flanges have been bent by an overzealous mechanic it may be difficult to get a reliable seal.
If I didn't mention this before, If you are going to use jet nuts, buy actual mil spec units from a reputable source like Pegasus or Aircraft Spruce. Good enough for actual aircraft is good enough for my Morgan.
