340 with edelbrock heads overheating

There are many possibilities here. Met rods jumping around at idle means low vacuum. So does the PCV behaviour.
Low vac could be cam installed retarded, wiped cam lobes or just a long duration cam, where low vac would be normal. Or valves not seating. Many suspects!!
If you remove the carb, I suspect a large amount of the t-slots will be visible below the t/blades [ more than 1/16"] .
This fits with the above scenario. This would not be a faulty carb problem, but a result of the above.
12* at idle with a big cam is NOWHERE near enough & will give above symptoms. But is it a big cam? PCV will also act erratically with a big cam. You can feel this by idling the engine with your finger on the PCV; if you feel movement [ vibration ] , then the vac is insufficient to pull in the internal pintle [ the PCV is just as valve that closes off against a spring ]; engine must have enough idle vac to overcome the spring & pull in the pintle.