Truck is a 88 D 100 with a 3.9. About April my timing cover has leaked. I tighten the bolts slightly on the water pump etc and made it worse. It leaks and then stops and starts again etc. About 2 months or so i bought some stop leak and it stopped. Put some oil in yesterday and saw it is leaking again. About 3 yrs ago had to replace the water pump and a few months later the gasket for the timing cover.I don't know what the deal is and am getting fed up. I found a truck with a 3.9 in a pull a part yard this weekend and may try to get the cover from it

 

Sounds like your cover may be warped from over-tightening. If you have access to a large belt sander you can see if its salvageable.