Boy, I would hate to see you put in a Chevy engine in an old Buick like that. If you want more grunt, put in a 455 Buick and have a blast with all the low end it will give you. Or if you wanted to stay in that period, you could might be able to find a 425 ci nailhead and put that in there. I have a 62 Invicta, and the best part of the car is the engine. Without that nailhead, it is just another car. That engine is what sets it apart from all the other cars. When I have it out, people want to hear it run and look under the hood.
Just a thought.