Most everyone has a different opinion on this ...
I think that the Pinto/Mustang II is too light a duty IFS ... for a big Buick.
They were designed for a small. lightweight economy car that weighed a least 2 thousand pounds less than your Buick. The track width and wheelbase are very different.
I had a guy in my area put a Pinto/Mustang II kit under a 1934 Nash. He had nothing but problems with it. The A arms would bend ... after a thousand miles or so. He could not keep it aligned. He had to get front springs so heavy rated ... to hold the front up that it rode like it had no front suspension.
A IFS from the Pinto/Mustang II is over 30 years old. Design wise ... better things are available.
Just my opinion
Deuce .... Moderator ...