Fast flight will probably last longer, but once B50 settles in, there is really very little stretch. Actualy creep, which is the string lengthening, is probably what you mean. The Dacron string will stretch a bit and recover after each shot. This is not a huge problem. Creep is what causes you to lose brace height over time. This usually stops after shooting in. Unless your string length is way off to begin with, twisting B50 to adjust brace height should not be a problem.
As far as faster arrows, I doubt you would be able to tell the difference without a chronograph. The only difference I have noticed is that fast flight type strings will shoot a heavier arrow without losing as much speed as a B50 string. If you are at 10 grains per pound of bow weight or less, the difference in speed is pretty negligible. Certainly not enough to warrant the risk to your bow.
Your bow was designed to shoot dacron. That's what I'd use. Don't risk ruining a fine old bow for the minimal advantages you might see with fast flight string.