1966 8 3/4 b body rear end

Vinson
The 1965-1967 year range will supposedly drop onto your springs if you stretch them upon installation--the perches are 1" wider in total.

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The B Body rear is posted as 59 1/2" drum-to-drum so you would be gaining a little more than an inch per side. I've seen cars use 90's Mustang wheels to address that extra axle width because the backspacing works with the skirted fenders of cars like yours.
I've even talked to a guy with an early-A Dart who did this, with a factory steel wheel, but it had to be skinny with a small tire. I don't know if it rubbed as he drove it!

David