Oil for a Slant Six that's been sitting a long time

You've got a lot bigger things to worry about than zinc level, which is more of a goose chase/snipe hunt than a real issue. You need to get the sludge and crud out of the engine without causing damage (as can happen if an engine is "flushed" too aggressively). I have good results using a cautious procedure involving homemade "soup". It will also be a good idea to service the oil pressure relief valve to make sure it's doing its job, as described in these two threads: thread 1, thread 2.

Once you've got the engine cleaned out, use a reputable brand of oil of a weight grade appropriate for your ambient temperatures; do not make the common mistake of thinking a heavy-weight oil is better. It's not, unless what you're trying to do is squeeze the last few miles out of a whipped/worn-out engine. If you're not in the grip of winter as we are up here, start with 10w30. I'm running 0W30 (zero W thirty) in my '73 Dart at the moment; I get fast oil pressure even on the coldest startups because I have chosen the oil weight according to prevailing temperatures. Be sure to use a quality filter (not Fram).

The engine mount's available from Old Car Parts Northwest amongst other sources, or you can probably still get in on the group buy for new poly '62-'66 A-body slant-6 mounts. The harness will also be easy, but I'll answer that in your other thread.