-Refit the expansion tank/reservoir and add coolant to the line if needed. Leave the cap OFF, and start the truck. Let it run to operating temperature, keeping an eye on the coolant level in the reservoir, add if needed, do not install the cap. Of course keep an eye on the gauge.
-Once it is warmed up, and you are sure the thermostat has opened, rev gently to 3000rpm and then gently let it come back to idle. Do this 3 times. Any percolating in the reservoir is air working itself out of the system and escaping "to atmosphere" via the uncapped tank. This is the air that causes the waterfall sound in the heater core if you don't get rid of it.
-Let it idle a couple more minutes, and shut it down. Let it cool down to ambient temperature (or as close to it as you have time to wait) and if needed, add coolant to the reservoir. NOW you can install the cap. Road test and see what happens.