Valve cover leaks are a pain on the 250. Remove and clean the cover, then straighten the lip and be sure to smooth out around the bolt holes. Also get the mating surface on the head clean enough to eat on. Get a cork gasket and a tube of yellow weatherstrip adhesive. It will need to be applied to both sides of the gasket and both mating surfaces, then allowed to cure per the label. I will usually do the head side first, and after the gasket is on the head then apply to the cover and top of the gasket. install the cover and torque to the manufacturers recommendation. I use an acid brush to apply the adhesive to the head, it allows you to reach everything.
For the transfer case, try placing an ad in the swap meet here. You can also try some of the 4x4 forums, hopefully someone here can recommend one. You can also try the
67-72 parts board there are a few 4 wheelers there.
Your tranny, if it's original, should be the SM420, reverse is to the left and down. Very tough tranny, preferred by the rock crawlers. It could just need an oil change. When I had mine out of the truck I drained the oil, filled it with kerosene and let it set a few days, rocking it every once in a while. Then I drained and put in new oil.
