Eaton Fuller makes manual transmissions, Allison makes auto transmissions. Both are good choices just depends on which type of trans is wanted. Eaton does make a remote shift cover for their transmissions that is required in a cab over/cab before engine truck. For a transfercase either a T138 or a T1138 from the military 5 tons here can be used.
Going off of the pics and a little bit of internet searching looks like the common class 8 truck engines/transmissions will fit and be the right size (10 liter up to 15 liter engines). Cummins, CAT, and Detroit are the 3 common engine manufacturers here. Budget and preference will determine which engine to go with but common ones are:
Cummins-855 series, ISX, M11.
CAT-3406, 6nz, C12, C15
Detroit-60 series
As far as the trans/tcase goes, need a measurement from the front of the engine to the end of the tcase, and another measurement from the back of the tcase to that first crossmember on the frame to find out how much room is available. Contact ZF and find out if they make a SAE bellhousing since that would be the easiest option, or Cummins/CAT are used worldwide so they probably make adapter housings.