Tom,
It has been about 20 years since I have worked on the 250 cu. in. engine but if my memory is correct that is an alluminum gear. If it is, then it can be removed while in the vehicle. First remove the radiator and pan and the front timing cover. You can then see what the damage is. If it is the gear, and all the other parts seem to be in good condition, drill and tap a couple of holes in the gear so a puller can be used to pull the gear off the cam shaft. Once the gear is off the shaft, and you have the new gear, then lay the gear in a shallow pan of oil and heat the oil till it is about to self ignite, remove the gear and align up the key on the cam up with the slot in the gear and the timing marks with crankshaft gear. If you do this quick enough before the gear starts to cool, the gear should slide on the camshaft easy.
