s6 to V-8 do I need to change torsion bars

Drag racers look for the thinner slant bars, to give better weight transfer to the rears, but otherwise thicker is better. Measure what you have first. There are charts to help tell what you have, based on the PN on the end or the colored paint marks, or just use a vernier caliper. Slants w/ factory AC had the same PN as 2-bbl V-8 (w/o AC?), at least in some years. However, even most V-8 bars are too springy for people today.