Can anyone explain the difference (ease of replacement) between the upper and lower manifold gaskets, and the Fuel rail to intake manifold gasket. The illustrations in my shop manual aren't all that clear and the posts here seem to be very helpful.

I have a 1997 Mazda B4000 4WD. I am also getting the 171 and 174 CEL codes. It stalls sometimes when stopped at a light and isn't running smoothly with the RPM's erattic at idle. The MAF sensor looks clean as a whistle. Someone (previous owner) spraid foam insulation all around the throttle body/manifold area at some point and I'm wondering if it was to cover up an air leak. I can't see any suspicious 'leaks' but I thought I'd ask you folks on the best way to proceed before I tear the manifold apart.