Oil leak Please help...

[Big 10]

I have an '80 chevy with a reman 350 that leaks oil and it's driving me nuts. The oil is coming out where the timing chain cover and oil pan seal around the cranshaft. The oil gets on the harmonic balancer and gets thrown absolutely everywhere. I thought it was a bad seal, but it only does it when I'm on the highway. It doesn't leak a drop around town, but the second I hit the interstate it will lose around a quart in 200 miles. All I can think of is a pressure build-up, but noone can tell me what's causing it. I already checked my PCV valve and the vacuum line attached to it and they look fine. I don't know what else to check... Any ideas would be greatly appreciated...

Thanks,
Eric

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'80 Chevy Cheyenne 1/2 350 4bbl.