I found an 18x9 +35mm wheel that I think will work. I can run a 275/35R18 on it, even though a 9.5 would be better. It was an open box deal at American Muscle so I bought it and then a second one so I have a set. The nice thing about the high offset is it puts the face of the wheel further out and I might actually be able to run the optional offset center cap with no machining.
So I am at a point where I could run a different rear wheel width if I still wanted to, and am leaning towards a 255/40R18 and 275/40R18 combo. I can run that combo with the 18x9, but there is an 18x10 +42mm or an 18x9.5 +35mm option in the same wheel, too. Current plan is a Crown Vic 8.8 narrowed to fit whatever wheel I pick with Moser axle ends so I can run '05+ Mustang rear brakes, eliminate the c-clips and keep the ABS tone ring.
But there is also an 18x10 +15mm option which might change the rear axle direction. Time will tell.
And the brakes are in flux now too as I bought my son's '73 Duster so now the G3/T56 is going in that along with the 18" wheels and (maybe) a fabricated spindle with a 2015 Mustang rear bolt on hub and 14.2" Scat Pack rotors and 4 piston Brembo calipers.
That's what I ended up buying, Forgestar F14's in piano black. I really like them, much lighter than the FR500 wheel I first bought. The only caveat is, if you buy them from American Muscle, they appear to have a deal where the center bore matches a Mustang while the wheel direct from Forgestar has a larger hub opening. Didn't matter to me since I turned my hubs to fit the Mustang wheels I run on my '74, but might be important to someone.
Forgestar F14 Wheels