And one other thing, and this is directed to Dan. The enging is getting really crappy fuel milage because the O2 sensor is seeing too much air, which makes the long term fuel trims drive rich all the way, or most of it. That's in the real world also, and as this truck has an exhuast leak, Lord knows how much air it's seeing. That's all assuming the fuel pressure is OK and it's not leaking raw fuel into the vacuum hose. All of which are fairly basic checks that should be done when any driveability problem is encountered.
And if you can identify the specific clyinders that are missing before you start tearing things down, you can limit your diagnosis to those area(s).
Good luck!