Small block without PS, and Air may work with slant six bar, but car would be a wallowing mush bucket.
Original slant six bars and V8 bars are too soft for good handling with any engine. If you plan on replacing torsion bars, a set at one inch and thicker will be a best bet on a closed car. With thicker bars, a good front anti sway bar, new rear springs, and sub-frame connectors, these cars can be made to handle and ride real well. Oh, don’t forget to install new bushings during an upgrade.