I would be more concerned about the 4L60e transmission than the motor.
I would drop the trans pan, change the filter, clean and reinstall the pan.
OReily's has the filter and gasket for $10.
Change the trans filter, and refill, then drain the pan and refill at the next couple of OCI's.
Use Lubeguard for good measure.
20/50 is to "heavy" for the 350 with summer approaching.
I would Seafoam the motor, then change the air filter, fuel filter, PCV, plugs, cap and rotor, and plug wires.
When you seafoam the motor through the PCV, run a half bottle of Seafoam in the crankcase and let idle for 10-15 minutes. Drain, and refill with 10-30 or HD 30 and use a new AC Delco filter.
The I would hit the rear diff, and transfer case. with drains and refills.
Finally, I would change out the radiator fluid, refill, and use a bottle of NAPA Kool.
Finally, suck out the old power steering fluid and brake fluid with a turkey baster and refill. Do this twice in one week (clean or use different basters).
Good Luck