If it has been taken proper care of, 91,*** miles is not a lot. Once you verify the PCV and crankcase breather system are both functioning properly, if you still think there is a blowby issue, you can try this product. Engine Restore.
https://www.oreillyauto.com/detail/...1967/plymouth/valiant?pos=2&manufacturer=true
I have personal experience with it. I have used it several times and it does everything that it claims. It comes in 4, 6 and 8 cylinder sizes. Only drawback, if you can call it that is, you must add it again with each oil change.
