Slant Six overheat at cruise speed, especially going uphill

Before you reverse flush, you need to get the bottom tank cleaned out.

Reverse flushing a system full of crud can drive that crud into the radiator tubes.

Run the system with the thermostat removed while running a cleaning agent in the system. Then pull the lower radiator hose off, and flush it from the top to get the lower tank clean first. It is the lowest point in the system.

.