This could be a lot things really. How long does it take to for the idle to start to climb after starting the truck cold? And how bad does it get, is it just ruff and high or will it stumble and stall at all?
What I'm thinking is that you either have a vacuum leak, an EGR valve that isn't closing quite all the way, or an idle passage in the carb that is clogged up a bit.
The best way to check for a vacuum leak is with a smoke machine, but most folks don't have one of those in their back pocket so next best thing is to inspect all the vacuum lines, and replace anything that looks like it might not be good any more (cracked or broken) don't forget that vacuum is run down to the charcoal canister so if there is a crack in the canister it'll cause a huge leak. The best way to test this is to unplug the vacuum line to the canister, plug it start it and see how it runs. For the manifold gaskets you'll want to carefully spray a bit of brake clean where the manifold meets the cylinder head with the engine running and see if there is a change in the idle. Be careful you don't want to soak the engine down and then have it catch fire.
For the EGR valve unbolt it, flip the valve upside down, set it on a hard surface and pour water into one of the passages in the valve. Leave it for a 1/2 hour and see if water seems past the valve into the other passage.
Plugged idle passage in the carburetor. This is going to require a rebuilt carburetor, either you rebuilding it, having someone do it for you or buying one. If the idle isn't to bad you could try running a bottle of seafoam though the next tank of gas. What would happen in this scenario is when the choke is on it makes it rich enough that you can't tell there is a plugged idle circuit, and once the truck warms up and the choke opens it idles bad. Sometimes this can be accompanied by an increase in RPM because of the leanness. Conversely if the choke is not opening up it'll stay on fast idle and maybe ruff because of the richness, but you did not mention the smell of gas or rich exhaust so I doubt that's the problem, but when it's warmed up if you look in the carburetor you should be able to tell if the choke is open.
If I had to take guesses I'd suspect a vacuum leak first, followed by carburetor with EGR last, but I'd check the EGR before tearing into the carb because it doesn't cost anything other than a gasket to check the EGR.
Since that was your first post welcome aboard! Since I don't know your skill level, I'll just say feel free to ask if any of what I wrote doesn't make sense.
