If you don't have access to an air chisel, you can also ground the heads off flat. Then use a punch and hammer to push it out the frame.... this could take a big hammer and some goos swings.
But it will work, and that is how I did mine. If you have the means, I would remove the bed, as it will make it a lot easier to work in there when doing the hangers and give you more room. Just have 3 of your buddy's come over to help lift the bed. The more buddies the better.
Btw, clean truck and looks like you have a great start. I like the orange. I think working on cars is a great hobby to be into.... and welcome to the forum!