Hi Hammerhead, The M49 bit is the part number for the ring itself, this common ring is used with several different types of support set ups, giving you the M36, M37 ring mounts. The M36 set up is correct for open cab CCKW, DUKW and some wreckers.
On the M36 mount, the M49 ring is supported by three steel legs, these legs can be adjusted in height in their clamp brackets to give a variation in clearance above the cab roof canvas. There is not much adjustment but there is some, maybe 2"- 3". Its worth noting that the height of the ring is under the height of the roof bows, its the traveler and yoke that adds the excess height.
I can measure the minimum height achievable above the roof bows for you tomorrow.
TM 9-801 however gives the following under Vehicle Dimentions:
Max Overall height with ring mount - unloaded 119 "
Max overall height Tarp installed (without ringmount) unloaded - 108.1/8" to 110"
......... depends on model.
Hope that helps.