I have run 18"x8" rims on my 79 Ranchero for 3 or 4 years. It is virtually the same chassis as your wagon. All 4 rims have 5 inch backspacing. the front tires are 235/50/18 with the rears being 245/55/18 which incidentally is the stock tire sizes of the recent but now discontinued Mercury Marauder.
I just this week received my 73 Gran Torino Wagon (purchased off ebay) and mounted the same 18 inch rims on it with no clearance problems. If you want 17s x8's, 5 inch back spacing shouldn't be a problem and seems to be a common spacing. It's hard to say if you could get by with 5.5 or 6 backspacing - at somepoint something will rub. 235/45-17, 245/45-17, and 255/45/-17 are all common sizes made by many manufacturers. I've been quite happy with my Goodrich KDWS 18" tires.
Want to play around with tires sizes and get specs? Go to
www.tirerack.com - click on shop by tire size. Enter in your desired tire and "Proceed". What ever returns from the search, click on SPECS and you can get widths and diameters for the various tires. Time spent educating yourself on tirerack.com is well worth it!
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