One of the great things about this site is all the different perspectives. If I had an original car that I wanted to "improve" but maintain, I would take the original drivetrain out and carefully save it for the next owner which opens up a lot of possibilities. My 70 Chevelle was already half in the bag when it was rescued, but I'm assuming the A-body floor pans are all very similar. The Tremec TKO trans did not require any tunnel mods and I didn't have to tub the rear to fit 295/35-18s. I would go at least 17" wheels to get into some good speed rated tires which will make a huge difference. Try this web site for larger but OE looking steel wheels--http://www.specialtywheel.com/wheel-showcase/oldsmobile.html.
Throw in a couple beefed up sway bars, a good set of shocks, mufflers that sing and you will have a real enjoyable ride. Good luck, have fun and drive that car.