Thanks guys, looks like Kakazu Ridge mortar caves is the way to go... I'd assumed it was a 320mm mortar and it looks like they were present in Burma. Source book is Ian Lyall Grants Burma The Turning Point.
Now to work in a scissors bridge and a bulldozer.....
'A Mortar Cave may attack only in a Friendly AFPh in which
all its occupants are Good Order, unpinned, and are using Opportunity Fire to operate the spigot mortar. Mortar Cave attacks are handled as OBA, with no penalty for being overstacked. The Japanese player places an SR in any target hex > 2 hexes away (LOS is not required) and makes an accuracy dr. An Original accuracy dr of 1 results in a direct hit, which is immediately resolved in the target hex as a 20 FP Indirect Fire attack. Any Original effects DR of 12 results in permanent malfunction of the Mortar Cave. An Original accuracy dr > 1 results in a near miss. Roll normally for direction and Extent of Error (
C1.31). All Extent of Error dr are halved (FRU), and no final target hex may be within 3 hexes of the mortar cave. Reroll any direction/Extent-of-Error dr that would do so. Near misses are resolved as a 20 FP Indirect Fire attack against the final target hex. In both cases, the Mortar Cave attack affects all Locations and occupants of the target hex
[EXC: C1.5; G11.84]. If operated solely by non-crew Infantry, the Mortar Cave's X# is reduced by 2 and the shot is automatically considered a near miss and Extent of Error is not halved. A Mortar Cave is revealed as soon as its occupants are marked with a Bounding Fire counter and a Good Order enemy Ground unit has LOS to either the cave or its entrance hex. If not so revealed, the Japanese player need only notify the American player that one or more hidden Mortar Caves are declaring Opportunity Fire. Mortar Cave occupants using Opportunity Fire are considered to be using Hazardous Movement, Mortar Cave TEM does not apply to such units using Opportunity Fire, but any TEM in the entrance hex does, and attacks vs such units may target either Location, both of which
must be in play.'