Seems early for mustangs, yours must get riper before the vine that we pick from. I do know folks that cook the juice out and freeze it and make jelly later, when it is not 100 degrees out.
We usually make jelly after the beginning of August. Depending on the year it can even be near the end of August.
We made 20 pints last year. That takes about a five gallon bucket full of grapes.
This thread makes me want to go check the vines, maybe it is an early year.
My wife does the cook and freeze for later canning method for Mustang grapes . We have family and friends that beg for jars of her jelly .