When I'm making gauge adapters for aluminum for a 22XX, I use a 3/8" center hole in the 3/4" OD insert and that's plenty of threads for a 1/8" NPT gauge fitting, provided it comes through just flush with the inside of the hole.... If your fill fitting has larger than a 3/8" hole where you will be tapping it, measure it and post here and we'll give it another think....

Bob