What shape are your rear springs? If they're still arched, I would just replace the bushings. You can steal them from the front control arms you've already replaced
the outer rubber parts are the same, just the sleeve is longer on the rears and you can reuse it if it's not gone.
If your leaf springs are flat and have no arch left, then replace them. HD or standard depends on your use for the cart, but without a lot of extra weight the HD springs are going to ride pretty rough.