250 inline 6 mystery oil leak

[68blackc10]

Hi guys I'm fairly new to the forum, so bare with me. I have a 68 c10 with 250 ci inline 6 and a 3 speed on the tree.
My problem is every since I bought it almost a year a good I've had a pretty good oil leak on the passenger side of the motor. I've tried changing gaskets on the valve cover and side covers, still leaked. Then I put a set of aluminum valve cover and side covers from a mercruzer motor with another set of new rubber gaskets, still leaks. So yesterday I pulled them back off and added silicone to the gasket, still leaks profusely. Is there something I'm missing with these motors? This is the first inline 6 I've own. It's hard to tell where it's leaking from because it coats the whole side of the motor from the distributor all the way back, and from the side of the valve cover down to the oil pan.
I was told the head was rebuilt right before I bought it. which it was real clean. Is there any plugs that might be missing? Cracked block? It runs real good so I don't think the head bolts are loose, but I should check them anyway.
Any help would be appreciated.
Jared Vicker