I tried this conversion in 1/35 scale many years ago - it's not that straightforward as you need a full 75mm and the gun shield is a complex shape. The M3 interior was revised too. I was hoping Rubicon would do one.
Ripley - the T48 GMC was built for the British, but by the time it was produced they didn't want any. So only 30 went to the British (and got converted back into standard half-tracks). 650 went to the Red Army, as the SU-75. Another variant I would have liked Rubicon to do.