If it isn't clear by now, you really need to free the heat riser valve as a priority. If stuck in the heat-on position you will burn valves pretty soon. IF stuck in the heat off position it will take forever to warm up and run well.
Heat the exhaust manifold with a torch being careful not to overheat it, then tap the shaft of the riser valve left and right rather than trying to twist it. Once overheated by a stuck closed heat riser, the exhaust manifolds tent to warp and cause exhaust leaks. Sometimes they crack. You're on the right track to have it refaced but do the exhaust manifold separately from the intake. Be very careful removing the bolts that connect the two manifolds together. They tend to crystalize and weaken to the point where they will snap off and then an easy-out will become your best friend or worst enemy depending on how it goes.
On old trick for stopping exhaust leaks on warped manifolds was to retard the timing and get it good and hot then snug down the retaining bolts for all you're worth. Doesn't always work though but usually worth a try.
When reassembling, leave the bolts that hold the manifolds together loose till last. Tighten the manifold-to-head bolts first then the manifold-halves bolts at the end.
