1966 Plymouth Valiant rallye wheel help.

The standard 15" Chrysler Rallye wheels are 7" wide with 4-1/4 back space.
The ringer here is 73-76 discs add track width. IIRC, 7/8" per side.

I have 15x7 LBP on my 68 with 73-76 discs.

I have 14x5-1/2 SBP Rallyes on my 64. I just had to replace 2 tires last week. Wow, the 14" tire selection is really bad compared to even 5 years ago.

So you might want to consider tire choices first.