If you would of stayed Olds V8, you could have used a 76 or older bellhousing. Olds used the dual pattern because of the Ford Toploader 3 spd used as the heavy duty option starting in 66 I believe and kept the holes there even after Muncie made their own heavy duty 3 spd in 71. I would assume the starter position would be an issue on the Buick. A member on here bolted those V6 trans to an Olds 350 with the dual pattern bellhousing, just the input shaft needed shortening. With an adapter you should not need to shorten the input. That trans is rated at 310 ft/lbs, the most of any T-5.