63 Dodge Dart, now let me see;)

Good grief! So much terrible information.

• A 255/60/15 fits perfectly fine on a 7" wheel. It's literally still in the recommended range from the manufacturer, which is typically pretty conservative (lawyers!)
• The 8 1/4 is narrower than A-body 8 3/4's, as already mentioned. It's a BETTER rear end for width on an early A.
• Air shocks are terrible. So are KYB's.
• It looks like there is PLENTY of room for a 255/60/15 on that car. You need a 15x7" with at least 4" of backspace. About the max on an 8 1/4 is a 15x7 with 4.25" of backspace, that's actually a mopar dimension, cop wheels are 15x7 and 4.25" backspace. With a 255 the 4.25" backspace will be pushing it for spring clearance, but a small wheel spacer is easier to add than extra room to the quarter!
• 255's are heavy. They will slow your 0-60 but once you're rolling it doesn't make all that much difference. If you're not drag racing it won't matter.
• 14" wheels do work with 73+ disks as long as they're the 10.95" disks and not the 11.75's. 14's were still stock on 73+ BBP cars.

I don't know who was doing the math on your engine RPMS, but, it's wrong.

With a 3.55 rear gear, a 26" tall tire (235/60/15), and a 1:1 4th gear at 3,500 RPM you get 75 mph.

with a 26.5" tall tire (245/60/15) at 3,500 you get 77 mph

with a 27" tall tire (255/60/15) at 3,500 you get 79 mph

3,500 rpm is perfectly doable for these engines. Maybe not what you'd want for an 8 hour freeway drive, but it works just fine and 75mph will be ok on the freeways as well. Heck with the 255's you're only at 3,200 for 72 mph, that's not bad at all.
You can play with all of that with the Tremec calculator mentioned above, but here's a good link for it Gear Ratio Calculator | TREMEC