On a '68, I've gone as wide as 15x8 steelies (4.5 or 4.25 BS depending on how you measure).
265/50R15 front, 295/50 rear, filled the openings nicely, didn't rub anywhere, actually plenty of room.
That 50-series sidewall looked a little too short to me, though nobody else seemed to notice.
Current '68 has 15x7 & 15x8 Magnums, 235/60 & 275/60, looks appropriate to me.
The 8s & 265/50s were a bit much parking with manual steering & Hemi.
The 7s & 235/60s not bad at all parking, manual steering & 383.
Member "mickm" here with a '70 seems to have tighter rear wheelhouses than my '68s.
15x10 with 5.5 BS & 295/65s is the most you can run on '68-'69 Plymouth B rear.