General tuneup for a 1970

[Jred]

Received my Avanti II yesterday and I’m planning on giving it a basic tuneup today or tomorrow. I’m pretty sure it’s running the Chevy 350 and 300 trans. Thinking of doing a coolant flush by draining the block via the drain plugs on either side of the engine and radiator, flushing with distilled water and using basic Preston coolant for final fill. Capacity is pretty straight forward, fill and run and top off as needed. 
The oil capacity is a slight mystery to me but based off a search in corvette forums and here I’m thinking 6 quartz will do. What kind of oil would you all use? The engine has 62k miles on it. I’m thinking straight dinosaur juice. I have Rotella 15w40 in garage from my landcruiser but that was a tractor motor with 200k on it and I’m wondering if it would be slightly too thick?

In short, what oil filter and oil type say you?

the cap, rotor, wires and plugs are all low mileage. The previous owner only put 5000 miles on it since 1984. Started regularly though. 
 

thanks in advance

[studegary]

I would not start with anything more viscous than 10W30.