Apparently the oil ring on the distributor or i should say o-ring seal on the dizzy housing is not the correct diameter to seal the housing.
First swap the o-ring seal from your old dizzy & see if it seals properly.
Using a quart of oil in 600 miles is excessive. I would change oil & monitor it for several more hundred miles. If the consumption continues two thing could have happened, the cylinders were honed with too rough a finish-280 & should be honed with a 500 grit hone. The 280 rough finish will destroy the moly on the rings & cause oil consumption.
Second what kind of valve guide seals were used??
If you remove the plugs & see a large amount of oil on the threads you have one of the above problems.
Back to the oil leak at the distributor, performance distributors builds a great unit for chevys, but what i've seen on the 200 six is sloppy workmanship on their rebuilt duraspark units.The DUI's ????
The loss of oil pressure when slowing down, sounds like the pickup screen & pipe came loose from the oil pump & is sucking air.
The 200 pan needs baffles installed to keep the oil in the sump even under hard stops.
The best example is look at a small block chevy pan with the trap door in the front of the sump to keep oil in the sump under hard braking, & also has baffles on the sides to prevent oil sloshing all over the place.
You may have to pull the engine & take it to a machine shop that knows what they are doing.
I hate to mention chevy, but they had there act together even in the early 60's- windage tray & all.
Remember this six cylinder back in those days was a mom & pops & grannys car for economy transportation, & now we are pushing the design over the limits. Bill