I think i'm in the minority on seeing it get nerfs on direct performance rather than late game utility ones.
Basically a combination of:
Slight cost increase
Reducing the gun range to be equal to other light cars
Giving Step it up a munition cost
i'd also rather see the early game performance adjusted than any of the late game abilities removed. kind of perplexing that so many people think what had been almost universally recognized as good unit design for light vehicles, i.e. lowered initial power spike but increased utility especially in later stages of the game, should not apply to the wc51 for some reason.
i'd just lower the long range dps and/or range and see from there. if that still proves insufficient a slight cost increase either for the unit itself or the upgrade might be enough to reduce its early game impact without nerfing the truck back into oblivion.