1968 GTO Wheel Swap

[1968 GTO Resto-Mod]

trying to swap out my stock 15" wheels for 18" wheels.

what is the maximum wheel width dimension possible on the rear end without requiring modification to wheel wells?

note: i'm running a stock 1968 pontiac 10 bolt rear axle.

any help is appreciated.
thanks!

[1968 GTO Resto-Mod]

thanks for the sticky note, camcojb!

[1968 GTO Resto-Mod]

check these out!
5 spoke rally II cast aluminum... 17" x 9" with 5" of backspacing!
they even come with the PMD center caps!
https://www.yearone.com/Product/wheels/spw179kcb