Anything eb s2 or after can fit 6" speakers. before that they were 4" holes in the front doors.
I had the same trouble with my ghia. I have 4" pioneers in there and they go alright.
Get 4's for the doors, 6's for the parcel shelf and a sub and your system will be sweet.
EA Fairmont Ghia : 3.9L MPFI, 2.5" cat back sports exhaust, K&N panel filter. Pioneer head unit, pioneer 4" three way fronts, Pioneer 6" 3 way rears, 2x 12" subs, blue interior light.
My main pride and joy : '76 XB Fairmont GS, 250CI, 140,000 Km and all stock.... for now! (Factory air-con on the way)