You have to take the transmission out and remove the mainshaft. The mainshaft (and transmission) must be completely dissasembled in order to get new syncros on. This is not a cheap or easy process.
The sm465 was put into trucks from late 60s up into the late 80s - in both 2wd and 4wd trucks. Don't worry about getting one from a 1 ton - all sm465s hav the same internals. Any 2wd sm465 will work. You should have no trouble finding one at almost any junkyard. It certainly will be cheaper than fixing the one you have.
