im assuming the teeth are ground off the flywheel? you shopuld be able to shine a light up in the access hole at the bottom and see if the teeth are torn up or not.. it could be your starter bendix is improperly shimmed or isnt punching out far enough to engage the flywheel.. or the motor is weak and wont spin hard enough to pop the bendix into the gear.
OK im looking at this from having swapoped a few trannies on school busses (3800s).. and not a truck cab.. theres lots of room to work from top and bottom in a school bus chassis..
at a minimum you have to back the transmission off of the engine. the engine will sit there without having to be supported, if the trans is an AT545 or allison 2000 series it should just be hanging by its bell.. if its a 643 or a 3000 it may have a side bracket installed just behind the bell on each side..
you'll have to drop the driveshaft
unless there ius lts of slack, take off the trans cooler lines
remove the dipstick
unplug the electrical connectors to the trans
rotate the engine and unbolt the 6 nuts or bolts from the torque converter.
if its hydraulic brakes, disconnect the park brake cable.
likely disconnect shift cable unless its super long
support the trans with a SOLID trans jack.. if its a 545 or 2000 series it weights 400 lbs.. if its an MD3000 series or 643 its upwards of 650-700 lbs!.
slide the trans back till the converter clears the housing (4-6"). the flywheel is most likely your flexplate on this engine.. unbolt it and pull it out.. it weighs about 35-40 lbs..
some homework you can do is take your VIN number and call a navistar Dealer.. give them the VIN and tell them you need a new flywheel.. they will give you an exorbinant price for a new one.. ask for the navistar part number.. now you can google the part number and find a good used one from TPI, Vanderhaags, etc..