[ls1nova71]

This is what I would do, but it's only my personal opinion.

Valve cover gaskets and grommets If they're not leaking, leave them alone
Intake gasket Yes, especially if you're pulling the intake
valley cover gasket No
knock sensors and harness Only if they look bad or if the wiring looks bad
Rear main seal You're doing this anyway
Rear cover gasket Only if it's leaking
Oil galley barbel If the truck has good oil pressure, no
Timing chain This one's a crap shoot, ask yourself, honestly, how many miles do you think you will put on the truck when done? If just a cruiser for the weekend, then I wouldn't worry about it
Front cover gasket and seal If it's not leaking, then no, but do if you change the chain.
Oil pump If it has good oil pressure, then my response is the same as the chain.
water pump and gaskets Gaskets if you remove the pump for any reason. But if it isn't leaking, no. Odds are it isn't the original pump anyway, and they are easy to change later if need be.
thermostat Same as water pump
spark plugs and wire set Look at them and see their condition. Again, odds are they aren't original either, and are easy to change later too.