Whoa -- a bunch of (fellow) CMC owners here. With a 87-93 Mustang rear, I use 265-50-15 on 15x8 rims with a 5" backspacing. The fronts are 225-50-15 on 15x7 rims with 4.5" backspacing.
This was after much measurement, and I like the way it looks, except that the rears would look a *tad* nicer with another 0.5" outwards on each side. What I don't like is that the rear tire is not tall enough to fill up the wheel well. I'll be changing them though.
Your Lincoln rear end should be about 3/4" wider I believe.
Because of the curvature of the tire on the rim, and the different curvatures with a given tire on different sized rims, it will be tough to predict what the end result would look like for your tastes. Best bet is to find someone with the rims you want, try one, and adjust the backspacing from there.
Hope I didn't confuse you more
Cheers,
-Neil.