Slick tire size inside arches on ‘67 Dart

Hi,
Are you running slicks snug in between rolled up wheel arches and leaf springs, please share some guidance?
I’m moving up from RWYB to bracket and want slicks in place of street radials, they are 245/60R15 on 15x7 rims, axle is non-std with discs.
What size and brand slicks fit in those ‘67 Dart wheel wells please?
I can swap rims to 15x8 and sort backspacing if it makes a better combo that fits.
My current guess is to swap in 26x9.50-15, is this good or poor idea?
360ci, 727, 3.55:1 rear.