Yes, the rear suspension is already a "4 link". There are a lot of options for rear control arms for G-bodies.
I had a nearly identical car to yours when I was in college: a 1987 Buick Regal Limited. Mine was odd -- it had power locks and manual crank windows. Mine also had the 307, but it had the overdrive and the optional 3.08:1 rear axle ratio, which made it pretty spritely around town. I took out the 307 and installed an Olds 403, which is a direct drop-in. I'd consider building a stout Olds 350 (different from a Chevy 350) and dropping that in. It'll be a direct swap and everything will still work.
Mine had the vinyl top that I had planned to get rid of. I wanted to turn it into a T-Type clone. I was gonna paint mine two-tone white over silver. Alas...life got in the way and I sold it to a guy in New York State. Mine was a NC car, too. Spooky.
G-bodies respond very well to body bracing -- just like the old F-bodies. Put a strong cross brace behind your back seat, if yours doesn't already have it. You can brace the rear frame rails behind the fuel tank -- that made a world of difference in my '84 Cutlass. These cars used what they called a "perimeter frame". It looks like a true frame, but it acts like a hybrid between a body-on-frame and a unibody. As such, the body provided some of the structure of the car, and took some of the stress as well. Cracks in the B-pillar area are common with high-torque engines due to frame and body twist.