I wouldn't waste time with SPART right now.
It uses 3 width and has a fairly high supply draw for a single battalion. So, this SPG might look decent:
View attachment 1050827
It creates a battalion that looks like this:
View attachment 1050828
It also requires chromium and tungsten to attain these stats spread out over 3 width.
Let's take a different approach:
View attachment 1050830
No chromium and no tungsten cost. Also significantly cheaper and has actual breakthrough. And more hardness.
And much lower supply draw. Most countries that start with IW tank tech can build this tank from day 1 as well. The improved howitzer requires quite a bit of research and doesn't come online anytime soon.
And it only uses 2 width:
View attachment 1050837
Now, because the cheap light tank can't mount the howitzer, it does have less firepower per width than the SPG version, but I think it wins out as a cheap AFV that I can just make with off the shelf tech in 1936. Slap it into some foot infantry divisions, and the Soviets should be able to literally put 1 of these in all 300 rifle divisions by Barbarossa. Those production lines are going to be running non-stop
for years. And I won't have to import tungsten or chromium for this design. (Which is relevant to a number of countries.)
If you can't tell, when it comes to tanks in foot infantry formations, I'm a firm believer in the following design criteria:
1) Can I build it using day 1 techs?
2) Can I keep the production lines running forever on maximum production efficiency?
3) Can I minimize resource usage for resources I lack?
4) Can I build it during 1936-1937 period as an expansionist country that "acquires" MIC from neighbors while I wait for better techs for real tanks and real planes? (Germany and Japan, I'm looking at you.)