You can not replace or even lap valves with the heads on the engine. The heads have to come off. If you bent push rods, there is a reason, and the reason, or the result will be inside of the cylinders. Not to mention, you need to see inside of all of the cylinders, and all of the positions need to be inspected as well. As far removing rusty exhaust, heat is usually your friend, as long as you don't set the truck on fire, LOL. You can always remove the exhaust manifold bolts, and leave them in place, without removing the manifolds from the exhaust pipe, but then you may develop leaks, at the donut gasket. If you lucky, you need to have a machine shop rework your heads. Worse case, your looking at a title rebuild. Worse case, if your on a budget, pull the engine out, and take it apart, your self, then take it to an automotive machine shop, to do all of the machine work for you. Make a few phone calls, to your local machine shops, and ask them, about reworking the heads, and about doing a total rebuild. Some shops will do the machine work for you, and you can do the tear down, and reassembly your self, if your in a budget, and feel comfortable doing the work, installing positions, cam shaft, heads, e ECT, your self. Ask them just how far, do they want you to take the engine apart. Then you can reassemble it your self, after the machine work is complete. If your not on a budget, then a crate engine is your best bet, if it's more than just reworking the heads.