T-BRR

Thia is my first time on this forum so I thought I would give you a little background on my truck. I've owned it for about nine years. Bought it with 86,000 miles on it, cheap, and in really decent shape. I put about another 40,000 miles on it and the engine was getting tired and the trans had a weak 1st gear. I bought a 1979 with a 94 or 95 crate engine. Swapped both the engine and trans in my truck and sold the 79 again. My original engine and trans are still on the floor of my garage. This crate engine has caused me many problems since I put it in last spring. My latest being an oil leak on the back of the engine on the passenger side. Very hard to see the origin. It runs down on the bellhousing and the starter. I just read a thread that said it would definately not be coming out of the head but I really can't find much if any coming out of the valve cover. Any words of wisdom here folks, (besides rebuilding my original stuff and putting it back in)?