now would be a REAL good time to check your valve guide clearance. get ahold of a dial indicator and magnet base and you can measure the valve guide wear to see in teh valves are wobbling around too much. If that's a 7.3 there'sa good chance that is the reason for the carbon. They went to positive seals on the exhaust guides which starved them of lubrication and causes excess wear. All the later exhaust seals were changed back to umbrella seals ( except for some of us like me who run synthetic oil, I installed positive seals on all the valves during a rebuild after fresh guides as I'm not concerned about the wear issue ) You can pretty well count on a few guides being out of spec, it's just how bad. If you catch them before they are too far eaten up, you can have false guides installed for a fraction of the cost of whole new guides if you have a good diesel machine shop. Then either get your valves resurfaced, or, my preferance which is to hand lap the things. ( i.e. stick and suction cup and two grades of valve grinding compound) you get a much better valve seat by hand than you do with a machine job, but it's not for the weak of constitution ( taking roughly 1/8" metal facing off 16 hardened valves can get maddening if you really aren't into it )
Make sure to straight edge the head for warpage and get a bottoming tap ( this is NOT a plug tap available at your parts store, you will have to go to a specialized industrial supply to find one of these or Grainger or Mc Master-Carr )and clean out all the head bolt holes in the block so you will get proper torque when you install the heads
