The Buick is 3″ narrower (measured at the outside of the cast iron exhaust manifolds) than a 5.0L Ford.
I’ve already tried this. That’s why I decided on the Buick 3.8L/TH200R4 set up. I wanted to build a 5.0L/T-5 car, so I spent a lot of time and effort gathering data and measuring stuff.
Using anything but the little C3 automatic or 4 speed manual that is in it now requires a lot of modifications to the floor (hump). Also, the bigger trans length is so much longer that the engine must be moved forward almost 10″ to use them unless a special tail shaft/tailstock is used that almost elimates it.
Also, using a stick T-5 or similar trans after modifying the floor hump, leaves no foot room for a clutch pedal. That’s why I was going to use an automatic.
The Ford 5.0L with aluminum heads (GT-40) and intake with a small Holley make way more power than these cars can handle in the form they were manufactured. The are not well engineered, stuff was just made to fit. You are still over 100 lbs heavier than a Lima four banger too.
Believe me a done up 2.3L Four Banger or a V6 make more than enough power for these cars.