Join Date: Jan 2015
Location: Injun Territory, 'Merica!
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Bob, unless your '66 was built extremely early and originally had a a late '65 production version of the"narrow" '64-65-extreme early '66 width Pontiac 8.2 width rear , the original '66 Pontiac 8.2 rear would be exact same width as the '68-72 Chevelle 12 bolt rear.
Adding 4 3/4" bolt pattern rear disc brakes only spaces the wheel mounting flange out a hair over 1/8" on each side. Have built or gone through every style of early GM A-body rear, 275-60R15 tire/wheel fit is possible with correct back spacing 15x8 or 15x8.5 wheels. The tightest fit I've ever managed to get a pair of 275/60R15's to fit with the later width rear was with a built up HD 8.5 A-body rear that went in a customer's '64 Malibu. Was very tight fit, so tight had to mount the tire/wheels on the fresh built rear & roll it under the car, then hook up the upper and lower controll arms, brake line, and bleed the brakes while the car was up on 4 HD jack stands. The '64 Malibu has quarter panels that extend well down into the area that would normally be open on other '64 GM A-body's. '66 & 67's had even larger rear wheel houses, just have to measure and make sure the section width is not out into the inner edge of the stainless rear wheel openings. Best to you.