I've not heard of anyone changing out the rear shackle mounts,but I suppose you could replace the shackle bushes with urethane style ones.
But as Cletis mentions why 'improve' it,they've worked well for 60 odd years......
If you rebuilt what's there with new rubber bushes,they will probably last for another few decades.
The only other problem I could see would be increased noise transmission as the urethane is obviously much firmer than the original rubber.
Something I discovered when using urethane engine mounts.....
