1969 Dart GT vert Cragar REAR wheel size help needed

1969 Dart GT vert- small bolt pattern- 8 3/4 POS
Currently running Cragar SS 14x7 all the way around with 235/60x14 BF Goodrich TAs

Need (4) new tires. I did a lot of searches on FABO- but still not clear on fit. Checking Cragar website and want to make sure Im on the right track.
Im leaving 14x7 on the front- the 235/60 just fit with occ rubbing when full car. May drop down to 225/60.

Want to get a bigger tire on rear. I was thinking of going 15x8 on back but worried about fit/backspacing/etc. Also what the largest tire I could fit without trimming wheel well? Theres about 1" space now (not equal both sides).

Was hoping 15x8 with 245/60 or 255/60 but have concerns... would I be better staying with 14x7? And will 245 or 255 fit under there?
Also the 14x7 and 15x8 shows available only in uni-lug in SBP?

Here are the specs:
14x7 SBP 4.125 BS
15x7 SBP 4.125 BS
15x8 SBP 4.25 BS

Any help or suggestions appreciated.