5.0L Intake Manifold Gasket ?'s

I have a 1998 GMC 1500 with the 5.0L V8, for its age it has low milage... just over 54k. However it is having a few problems growing old, a few of which I can handle fine myself, and one biggie that I am going to get help with. Over the last few months been noticing a very slow leak in coolant, and noticed a small leak at the bottom of the thermal overflow tank, repaired that myself, then took it to the dealership for a more detailed leaktest and coolent flush. They found that the intake manifold gasket is leaking, and wanted over a thousand bucks to fix it. I am living on student loans right now, and won't see ANY kind of income until at least august, and a very small one then, so I had to wait an hour for them to find the keys to my truck and drive it on home unrepaired. A friend of mine is a Nissan mechanic, but he has done the intake manifolds on several GM/Chevy pickups before stating it is a fairly common issue (something about the gasket reacting badly with the dexcool). I also have a bit of leakage around my intake manifolds, sounds like a bit of clatter on the left hand rocker assembly, and a stuck thermostat. I am going to do the valve covers and reshoot the lash on all rockers myself as well as the thermostat while I am at it, but wondering WHEN i should do these repairs, will any of these parts need to be removed (other than the thermostat/water inlet area) for the intake manifold job? Looking through Chiltons it doesn't mention a need to remove the valve covers again, but looking at the truck it seems that it may be needed to gain access to some of the bolts. Ideas?? Suggestions??