I've been trying to find this out. The Long Tom is listed as NA (pg 405, Tech manual) for crits on Mechs, and no Mech Generator Program I have found allows it. The only question I could find on this was dated 2011, and was answered that no Biped Could but that the rules for Quads and Tripods had not been written yet.
Considering that Artillery is woefully underpowered in TT to begin with, even the prohibition (if it were ever official) on Long Toms seems very much like a carry over from the standard Mech rules, where the Split Critting simply was not possible due to number of crits involved.
So, as the Subject says, can Super Heavy Mechs mount a Long Tom Artillery Piece, and if so, what limitations, if any, exist?