Who else here hates mixing aluminum and steel fasteners? Well, First the bike is a '74 Z50 not that that really should matter but it may. Well, okay First there is a heat shield on either side of the head of the pipe, the two screws closest to the head will not budge for the life of me. There how ever are not recessed and can get vice grips on them. After soaking in some penetration fluid I grabbed some vise grips and got the first screw to turn about a 1/8 of a turn, being extremely careful to not break it. Well, of course with almost no pressure on it at all it broke off clean with the nut thetas welded to the pipe ugh, I should be able to get that out with an easy out. . .I hope. On the opposite side no matter what I do the screw will not budge. The head was already pretty mangled (who ever had this bike before managed to strip just about every nut and bolt on the thing). I tried spraying it down and letting it soak, locked some vise grips down. . .no luck. I then grabbed the torch and was careful to try and only heat the nut welded to the pipe and not the screw, then tried again the thing will not budge for the life of me. Its like it welded its self in place or something . Now for my second problem, the screws holding the other heat shield are also stuck. The heads are also stripped, and to make it worse they are recessed into the heat shield (and one with an older z50 knows what I mean). I used my dremle and was able to get a decent slot in the head and use a large flat head on it, with no luck. I've tried all I can think of and heating wouldn't work as I can't get to the nut that it is threaded into. I'm lost here and have run out of ideas, what should I do?!

Sounds like you are on the right track, keep spraying them down (every day) and keep working on them. Once you get them to break loose, try going back and forth a little bit each way and it should come out a little further each time. I'd say on the recessed ones you are probably to the point of using an ez out on them. If all else fails try to drill or grind the heads off completely, remove the heat shield and go to town on the leftovers. Good Luck

Allright, I'll have to pick up some more spray or would wd-40 work?

Id try some pb blaster. it penetrates pretty good. or soak the engine in some kerosene or something. by the way all of this sound you might be better of with a good set of drill bits and some taps. your gonna put some newer bolts in there anyway you may as well have the size that you like !!!!

Some general stuff on EZ outs....
the most common design is of an anticlockwise threaded tapered type. These rely on the EZ out 'jamming' in the hole you drill, enough to turn out the stuck bolt. What often happens though is that the taper just spreads the end of the bolt, and jams it even tighter in the hole!
Snap On (or Blue Point) produce an alternative type, where you still drill a hole, but then drift in a fluted straight bit that you then slide a hex coller over, allowing you to use a spanner. I have a kit like this myself and it works really well on 90% of stuck stripped bolts.

The coefficient of linear expansion is greater for aluminum than steel. Would a person be better off to heat the entire assembly up some taking care not to damage seals and such?

Well, I've used 2 cans of PB blaster and it seems like it did nothing at all lol. tried heating it several times, nothing. Tried tapping in all different directions to try and loosen it, didn't work tried an easy out on one of the screws and it did nothing but make it worse if possible. Sully I know what you are talking about and have used that type before, wish I could afford a nice set of them. Looks like ill end up drilling them out and re-tapping the holes. Any other Idea's? Man these screws seem to have become part of the friggin exhaust.

I've seen someone use a left handed drill bit successfuly before.