As far as I know the 66 B body Pontiac rearend is exactly the same as 65...axles shafts from 65-70 B body are fully interchangeable..I have 1968 Catalina shafts in my 65 housing right now. Rearend width is 65.50" inches brake drum face to brake drum face.
Lower shock bolt mount location unchanged as well, at least between 1965 to 66.
I believe backspacing and offset are confusing terms, so to clarify, if you take my modified rear rims, without a tire on them, and lay them face down on floor, set a 3' or longer straight edge across back side edge, and measure down from it to the point where rims will contact brake drum, that distance is 5.50" .....and if ya measure down to the ground through one of the spoke holes, that is exactly 9". (which means a 15 x 8" rim is actually 9" wide overall, and 8" wide bead to bead)
What I di was cut most of the "ring" off a pair of 15 x 7 Rally II, leaving only the centers, shipped them to Stockton told them to put 15 x 8 rings on them with 5.50" backspacing. They shipped them back, in bare steel, for $100 each, plus shipping. Really, from the outside of the wheel, they still look like stock 15 x 7, as far as "depth"...because even though the rims were widened 1 extra inch, it is all to the inside, therefore adding 1 extra inch to the backspacing as well.
If you modify your rims like I did mine, there is NO WAY you can retain the factory fender skirt latches. Even stock 15 x 7 Rally II will rub the tire sidewalls on the latches.
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