Ok I thought about this more.
Before you go mad.
You can do a few things here, rent a coolant pressure tester from your auto parts store and manually pressurize the system and look for a leak.
If you can't find the leak but the pressure goes down, remove the spark plugs and look in the cylinders.
Still can't find it?
Borrow a leak down tester. This tool uses compressed air. While the plugs are out you set whatever cylinder you're testing to top dead center. Then screw in an adapter to the spark plug hole. Remove the radiator cap. Using the air regulator on the leak down tester, turn it up until you have 100 psi in the cylinder. Look for air bubbles coming from the radiator, this will tell you if the head is cracked or the gasket is comprised. Repeat on every cylinder.
Or just have a decent mechanic do this test for you. Maybe they will let you watch if you show up with bagels and smear for the guys.
But seriously. Ls swap. It's like 5k for. 5.3 swap and you will be very happy, the value of your truck will triple.
Don't sell your truck for 3500.