Excessive Blowby Advice

[qbrew]

I'm trying to resurrect my old 77 Chevy C10 Long bed 350/350. I finally got it to a mechanic to repair the rear-main seal and he said it has "Excessive Blow-by, and the aftermarket blow-by filter was plugged. He advised me that the new seals and gaskets may blow again, so my parts and labor doing so may be wasted.

Should I go ahead and have the Rear main seal replaced knowing there is excessive blow by, or should I just put that money toward a motor? The motor was pulled from a running vehicle and was supposed to be good and running. It's not looking like that was the case.