I think it is possible to make a 55 ton mech (for some reason I am imagining a shadowhawk) with a thumper artillery piece taking up a large portion of both its left arm and torso. The mech would use a 225 engine (why 225 because you will need all the heat sinks in the engine and the 220 requires one heat sink outside of the engine), endo steel construction, null signature system, angel ECM, 4 jump jets, 11 tons of armor, double heat sinks, case, and 2 tons of ammo. For point blank defense you get 3 medium lasers (2 in chest and one in the head) and an ER flamer (right arm) to take out infantry that try to get close and potentially light things on fire for utility.
The succession wars variant would have only 10.5 tons of armor, no case, no ECM, normal medium lasers, no null signature system, single heat sinks, and a standard flamer. It also would use the more typical 22 engine since critical space is no longer a problem.
I think in both cases they would be a very solid option in their era for artillery.