LT1 flush
The service manual says flush with plain water only.
Pull the knock sensors and remove the lower hose at the radiator to drain completely. Fill, run, drain a couple of times and it will come out clean. Watch the analog gauge and make sure it does not get up to 260. Without antifreeze, you will likely get boiling at 212 so don't run too long during flush.
There should (may???) be a flow control in the feed line to the heater core. It tends to plug up and is easy to remove and clean so it's a good idea. A new one is only about $20. While it's off, flush the heater core separately. There's only 15psi on the core while driving so don't apply city water pressure or the core may pop. Also, the core nipples are fragile so I like to take the hoses loose at the 'other' end to flush the core.
And above all, don't get the opti-spark distributer wet (it's under the water pump).
For the last fill, there are brass air bleeders on and under the thermostat housing (keep the opti dry). After the final fill, the service manual recommends cycling the temperature full up (190) and full down at least 3 times before 'getting on it' to make sure all air is purged. And watch the analog gauge during this burping or purging to make sure it remains under 260F (the temperature that 50/50 water/antifreeze boils).
Your year would have had 'green' antifreeze from the factory.
Hope this helps
