On my 216 the rear seal doesn't start to leak until the engine has warmed up a bit, then it leaks more at idle than at higher rpm. It'll continue to drip some after shutdown also.
My rear seal leak avg's around 1 qt every 200-250 miles.

Is it possible you had the oil level overfilled initially?

Using a mirror and light, checkout the valve cover to head gasket mateing area's at the back of the head between the cover and firewall. That's the lowest spot for return oil and sometimes the oil can build up there and find it's way out via a bad gasket seal. My 216 seeps back in that area after a good hard run.

Also check the side cover rear bottom corner where it mates to the block. If that gasket has a leak you'll have a pretty good stream of return oil from the head to the oil pan.

Once oil warms up and thins out it'll seep out in all kinds of places.

You also mentioned your draft tube may be clogged and could be causing the bottom end not to vent. That's a good possibility and easy enough to check. I've seen engines spew oil all over the place from all sorts of sources when someone has buggered up it's venting system. An engine's got to breath!
The draft tube is pressed in so you'll have to knock it up to get it out of the block. Be careful as it's easy to deform it. You could try to blow compressed air back up through it, you should hear air hitting the oil plus escapeing out the valve cover up top.

Good luck
Dave

1949 Chevy 1/2-Ton 3100
1949 GMC 1/2-ton 100
In the Stovebolt Gallery
More pix on Photobucket
Video of the 216 running
Video of a Winter Cruise