The IRS will handle better and leave you more room for error. Typically the trailing arms are narrowed 1 to 1.5 inches. Airkewld and a few others make rear disc brake conversions that add no track width, so you can get a 185 section tire under the rear fenders with discs, and depending on the tire maybe even a 195, but that's going to vary a bit from car to car given our plastic fantastic bodies.
You can opt for a sway bar or not, I have opted to run one. The difference in feel is noticeable, a more planted sensation at higher speeds and on bumpier surfaces. If you're running a wide body, it's a no brainer, go IRS.
All that said, these are old tech cars and they require skill, patience, and concentration to drive fast. For most of us, that right there is a big part of the charm.