Are you near the coast? If so, the salt in the air likely rusted the two dis-similar metals together. In extreme cases, the two pieces of metal will
never come apart. I still have my orig FCA and spring, and $1000 challenge for any fool that thinks otherwise.
IMHO, I suggest just getting another NEW MC. It's seems like a good time to replace it. For the prop valve, those fittings are more more likely to be "one forever" with the prop valve. When I did my 5-lug upgrade, I cut lines off a number of prop valves in the junkyard. I could *melt* the whole thing and the fittings would still be one with the prop valve. Note that I'm not some person that's clueless about metal/heat properties. My minor was in Mech Engr.
With the above said, heat is the best way to get fitting off. But, that WILL destroy what ever the fitting connects to.
In cases where I can't use heat, I use the "Vise Grips for nuts" for hard to remove/rounded fittings/nuts. One of the MC fittings on my Olds is rounded off badly. But, that Vise Grip still gets it on/off.
IMHO, no tool box is complete without a set of the following Vise Grips:
Vise Grip 4LW
Amazon.com: Vise-Grip 4LW Locking Wrench with Wire Cutter
Vise Grip 7LW
Amazon.com: Vise-Grip 7LW Locking Wrench With Wire Cutter
Vise Grip 10LW
Amazon.com: Vise-Grip 10LW Locking Wrench with Wire Cutter
The Vise Grips will
very likely do the job!
But, as I said, there is that slight chance that the fittings, and what they connect to, will have to be replaced.
Good Luck! 
