318/390 stroker kit oil leaks NEED HELP!

Nice looking engine compartment! And how cool is that for your father!!

As others have told you, this sure sounds like a PCV issue. Under load, all engines have some blowby that they don't have when you simply rev them up. So unless that blowby has somewhere to go, it builds pressure in the crankcase. And it will find the weakest sealing spot in the engine to escape and push oil with it. Production pcv systems are designed to maintain a slight depression on the crankcase. They do the "suck" and the vent on the opposite valve cover provides the fresh air to the crankcase. These systems do this pretty well until us hotrodders come in and throw new/different parts on them not necessarily designed for them. The pcv valve itself is designed and sized for an engine. A "universal" aftermarket one may not be proper for your engine. I highly recommend you try a more prod type of pcv valve - just get one and plumb it from the RH valve cover to the full manifold vacuum port on the rear of the carb. It doesn't have to look great - just do it as a test piece and see if it helps. (Note I was not using one on my 408 and was using header "extractors" on both sides. When driving the car on the street, I noticed just a weep from the front crank seal and maybe a tad from the rear main. Put a pcv valve/system on it and....no more seepage! Just sayin'.....) Good luck with your project!!