I am with those who are siding on the valve problem. I would recommend an entire valve job since you are going to have to remove the heads to "clean" them anyway.
If I recall, since it has been a long time since I lifted valves out of my V-8, you will likely have to remove the manifolds, as well.
You will need a valve lifter. It is a special tool that compresses the valve spring while you lift out the valve itself from the block.
You can use the present valves so long as when you grind away the carbon deposits, you don't reduce any valve tolerance. Otherwise, go on ebay and find a set of valves...They are likely there.
Btw removing the heads you can also inspect the piston heads as well. This will tell you if the engine has ever been rebuilt. You may see a .40 stamped on the heads of the pistons. If I remember correctly, this means the cylinders were bored out 40 thousands over the initial bore. You would need this information if you replace the piston rings. Do everything that you can while the heads and manifolds are off, you won't regret it.
Btw, be sure to remove the head bolts in the series recommended by Ford. You should get a Ford manual for this since if you don't remove and put them back in the proper sequence, along with the proper torque, you will likely ruin all of your hard work.
Here is a link for the engine manual:
http://www.amazon.com/Complete-Ford-.../dp/1884089119
Here is an illustration of the V-8 FLathead:
http://restoreyourford.com/v860parts.shtml
Good luck