what to do with the six

Welp, Don't want to get you discouraged, but there is a ton of documentation on the slant over at .org

Engine FAQ

Slant Six articles

The biggest issue is going to be where you stop doing stuff to it :)

General tune up stuff will be a good start but have a plan first. For example you don't want to spend money on a new this or that if you will be upgrading it shortly there after.
Some things for consideration are:
Valve adjustment
Advance the timing - 5-8 degrees BTDC
Fuel line mod - if it is having hard start when hot issues
NGK long nose plugs
HEI mod - better idle and starting, allows you to remove the ballast resistor if you use a 12v coil as well
Electric fan or fan clutch - to get less drag on the motor.
2.25 inch exhaust.
Super six setup - 2bbl carb and intake
Gasket matching / head work.
....and the list can go on for weeks... but like I said, it's just when you want to stop... :toothy10: