I've long been fascinated by that upgrade, as well. They seem like a very efficient and cheap way to get modern self-propelled 155mm systems.
As for the Soviet 122mm M-30, that's an efficient little gun, isn't it? Again, in general I sort of keep "families" of comparable guns in my mind. Also, remember that a howitzer is usually lighter than the equivalent "gun" (Kanone) artillery for the same caliber. For instance, a 155mm howitzer is lighter than a 155mm gun or 155mm gun-howitzer. Also, as a general rule of thumb, US artillery is a good middle-of-the-road baseline, Soviet artillery is usually lighter (because it is generally of newer design), and German artillery is usually of slightly higher performance, but heavier as a result. Obviously, as the years go by, guns get lighter for the same or better performance.
Theoretically, you could put a Soviet 122mm M-30 in whatever vehicle used a German leFH 18 or a US M2/M101 105mm howitzer. So, a practical upgrade would be a East European or Middle Eastern modification of a Wespe or Grille with a 122mm M-30, instead. I can particularly see Yugoslavia, Syria, or Egypt using such a vehicle. As an example of the equivalence of these weapons, look no further than the German modifications on the Lorraine-Schlepper. They carried the German 10.5cm leFH 18, 15cm sIG 33, and Soviet 122mm M-30 on the same chassis.
You could also modify the Sexton, M7 Priest, or the various AMX Mk 61 equivalents with the same weapon. I think an interesting vehicle would be an open-topped, turreted self-propelled M10 or M36 tank destroyer, but with the gun replaced with the postwar Soviet 122mm D-30 and used in the indirect fire role. Like a poor man's 2S1 Gvozdika. You could also replace the old French 155mm GPF on the M12 with a Soviet 152mm ML-20 gun-howitzer.
I also think something along the lines of the Israeli self-propelled artillery Sherman variants has a lot of potential.
You could take a vehicle like this and put all manner of guns on it. The Soviet ML20 152mm gun-howitzer, postwar Soviet D-20 152mm gun-howitzer, D-74 122mm gun, M-46 130mm gun, or even Bull's fabulous 155mm GC-45 variants. This same range of weapons would obviously apply to similar post-war SPGs like the M40/M43, M41 "Gorilla", and the aforementioned M44 and M52.
The M37 HMC's layout makes it a bit difficult to put anything too big in it because most larger guns would be incapable of high angle fire in the standard mount. The Soviet 122mm U-11 would work, as would the German leFH 18/1, but there's really not much else of the same caliber or larger. I have long thought, however, that it would make for a good self-propelled mortar vehicle, like the experimental T37 4.2-inch SPM.
It wouldn't be hard to convert an M37 to this configuration and it could easily take a 120mm mortar and possibly even a 160mm mortar.
Cheers,
Logan
