Both are H platform cars, Just about everything under the hood should interchange. Before changing transmissions verify correct gearing though.
suspension should interchange, but the Buick is going to have much softer spring rates then the Bonneville. When I repaired my ELC most of the parts I used came off a few LeSabres at the junk yard.
Some interior parts should interchange, what will will be very limited, All I know that works is the the plastic strips that cover up the divide on the front door windows do interchange.