66 Dart wheel size

so do you guys think a 17x8 with the 4 1/8" back space is too wide? maybe the 17x7 would be better if i want the rear lowered? ill go as far as trim the fender lip, but dont plan on moving springs. itll be awhile before i need rims, so ill have plenty of time to research. posts like this help out alot.
hold on......i thought about it for a bit. the 4 1/8" backspace would be the same on an 8" rim as on a 7" rim right? back space is from the front of the drum or disc back right? and the back spacing is the same because thats the furthest back/inward your rim can go before its too close to the springs right?
that said, does switching from drum to discs throw this back spacing measurement off? because i plan on putting disc brakes on my 8 3/4.

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