68 FB Help

OK Long story here. I recently had some engine work done on my 1968 Firebird 400. While replacing the clutch I decided on having a new rear main seal installed. What started as a simple job became an almost complete rebuild. Here is the problem when the block was sent to the machine shop the cylinders showed an average of .007 wear. The machine shop recommended new pistons as well as a cylinder boring. Do to being overbudget at the time I had the cylinders hones instead and new rings installed. The cylinder heads had no valve work done other then new springs, and valve seats. The camshaft in the engine was replaced with a Summit 2801 with new lifters and pushrods. I also replaced the valve studs and locks with BB Chevy style in order to adjust the valve train for zero lash. My problem is this the #8 cylinder is leaking oil from the mating surface of the exhaust manifold. The engine also has a steady miss in it. Any ideas here? Also how do I fix it? I don't really want to tear the engine apart again.
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