noob needs help with mileage - only 10 MPG!

GDAY .
Regarding the knocking noise at warm up. only at idle
The same symptoms were occuring to my /6 recently.
I could never get the engine running 100%
I pulled apart the engine and the cam was cactus.
One lobe was very worn along with the lifter.
others were starting to wear.

This may be what is happening to your slant. It would explain rough idle( an inlet valve may not be opening) that cannot be fixed by normal tune up ,carb fix etc.

Along with those leak down numbers( assuming the valve adjustment is correct) it appears an engine re build/replacement may be needed.
I hope I am wrong,as I realise you probably dont have the skill/knowledge to do this yourself.
A second hand engine replacement may be the cheapest (not best )option.

Definitely get a set of feeler gauges and basic tools( sockets spanners) and learn to do the valve adjustment.
Check the list in this link for "valve adjustment procedure"
http://www.slantsix.org/forum/viewtopic.php?t=38446
lots of good reading in that list/site.

Good luck.
regards .
Brendan