noob needs help with mileage - only 10 MPG!

As all the others have said, there's lotsa possibilities. The smooth-when-cold and rough-when-hot scenario w/poor mileage sure seems to suggest a too-rich mixture. As loose as the engine is, you may bever get 20+ mpg until you do an engine overhaul. Bad rings won't generally cause rough running, but if you have some cooked valves, along with uneven carbon build-up in the head, you may not be able to get adequate compression in the cylinders for good mileage and a smooth hot idle.

An NOS Carter BB carb is not cheap, but lots cheaper than a rebuild. So if you've tried all the suggestions, it's the only one left I'd put money in. The good up side is, even if it does not cure the problem, you will have a quality, high mpg carb to use after your engine re-build. A quality factory rebuilt long block Slant 6 from a good rebuilder is not the end of the world price-wise; like, way less than a year payments on a new car. Since your car is pre-smog, you can do the engine swap sans the smog stuff you may have inherited on yur current engine. 20 years ago, I'd have suggested cruising the junkyards for a good salvage engine (probably what the previous owner did) but the chances of finding a tight, low miles used stock engine at this point is pretty tiny.

Good luck