They tried that and it was a dismal failure:
https://en.wikipedia.org/wiki/Panzer_III/IV
well, that was just a 1944 paper project, so hard to call it a failure at that point of the war. To me, at least one would have to been built for it to fail
The previous attempt had been the Geschützwagen III/IV, which had serious issues meshing the components.
this was closer, but more with the aim of making an actual gun tank rather than both III and IV chassis.
A Mk IV with torsion bars, if nothing else. pick one design, and make lots of them
I think there were more issues than just turret ring size; the overall weight of the new gun pushed it over the limits of what the chassis could handle.
The Brit Challenger did the 17 pdr on the same
1650mm size ring, but added an extra road wheel station for the added weight.
More like that, not looking for a Panther level of protection that led to the 'too much turret for the chassis' when the Germans tried the panther turret on that chassis
The Panther mantlet alone weighed near as much as the entire KwK 40 gun used on the regular Mk IV
I'm looking more a move to what the US TDs(and postwar AMX-13), reliable, very mobile lighter weight tanks with a hard hitting main gun.
They needed numbers and reliability.
By late 1942, unless there was at least effectively 3.5" of armor protection from both thickness and slope, you were going to be penetrated at under 1000 yards.
So why bother with that weight of armor that won't protect you? Have some breakthru tanks with heavy armor, but have tanks that can exploit and move rapidly without breakdowns.