You in my opinion can do some homework if you wish and find out the intended pressure release point of that valve when these cars where new. In my humble opinion if you gain any more than 2-4 miles per gallon you are too far over pressure. It is very hard to get a handle on fuel milage with any accuracy.

Yet two to four miles per gallon should be kind of felt intuitivly if there. By the same token if you are getting less mpg. Not a great probability . Remember to try to drive the car in a simular fashion to what you were before the stretch. Power tends to corrupt remember. You would sure notice it.

The best approach would be to tee a fairly accurate pressure gauge into the line somewhere past the lift pump and see what you have. Do not read the pressure before the filters if you feel they may be comprimised.

For the present people should not all rush out and stretch their relief springs. Yet if your milage is substandard to the good examples of your type. Reading the injection pump residual pressure might help establish a problem exists. It's pretty early with this yet promising.