Re: Oil in the air cleaner?
There are caps that will fit the tube and they have breather holes at the bottom.
I think you should solve the problem though, because I don't think your carb would be trying to breath through the manifold if it had free air flow through the cleaner.
It sounds like you retained the original air cleaner and it may be that the vacuum operated diaphram/s may not be operating correctly to allow proper air flow. I believe they are controlled by the thermo switch which routes the vacuum to open the diaphrams which adjust the vented air flow.
For all that you can certainly switch to a K/N filter, but you'll loose your exhaust manifold heat (which is nice in the winter if it gets very cold where you live) and be more likely to consume water that may fly around your engine compartment on fordings. Also, you can't fit a very tall K/N under the hood without a body lift, I believe my filter was about 2" tall.
Also, be sure that your pcv is functional and clean.