Based on the bulge of the tyre, I'd say a 7" wide rim. If it were an 8", there would be less bulge.
The 255s look good, what size rim? Are they on all four corners or just the back?
Have always thought a lot about wheel/tyre combos, for a Fusie C-Body a nice size (without looking too out of place) is:
Front: 235/70R15 on 7" wide rim
Rear: 275/60R15 on 8" wide rim (4.5" BS)
Based on mathematics, overall diameters are the same so would look nice (235/70 tyre with 164.5mm wall, 275/60 tyre with 165mm. Again, this is purely based on calculation, am sure would vary depending on tyre manufacturer).
Have also thought of (sorry, am a muscle car man at heart lol):
Front: 235/70R15 on 8" wide rim
Rear: 275/60R15 on 10" wide rim (not sure on BS measurements here. Depends obviously on the year of car).
For a C think that the 295/50R15 tyre overall diameter is just not large enough. Too much gap between the wheel and arch.
Remember that the 1978+ Trans Am with the WS6 suspension package came with 8" wide rims with 225/70R15 tyres (similar in overall diameter to a 255/60R15) but I personally think that the 225 on an 8" rim is too narrow.
That black '72 Dodge just looks downright mean, good wheel/tyre combo. Any idea of what that lad has on there?