If you're asking which T-55 would make a proper load for the MAZ, I think any T-55 variant that was in service from the middle of the 1960's on would be just fine. So you could use the Trumpy T-55 Model 1958 (with a little, or a lot of, work) and be reasonably accurate.
I'm not sure when the MAZ-537 went operational, as I couldn't find any exact dates online, but all sources that I've seen say the mid-60's.
I'm sure one of our local experts could straighten me out on this!
-YL