When was the last time the plugs/wires were changed? That could account for the misfires. These coils are known to be pretty solid but they can cause issues. The ICM (Ignition Control Module) that the coils sit on (the large metal rectangle under them) can also cause issues. Also, that many misfires will destroy a cat converter so be gentle on it.
The EGR codes are probably related to a bad purge valve solenoid or possibly a leak at the EGR. The flexible tube on the EGR like to break in the accordian sections.
If the Crankshaft position sensor went completely out the car would not run due to it not feeding the ECM the info it needs in relation to cylinder postion, firing order, etc... If the car still runs but you notice the tach dropping to zero before it dies out then that would be a tell-tale sign of a bad Crank sensor. FYI- You have to remove the harmonic balancer to change out the crank sensor. It is located behind it on the right side.
The P0420 could just be a bad O2 sensor. If you suspect a clogged cat then you could test it by either removing the O2 sensor and see if it runs better, or drop the exhaust off of it and test. If all is well then your cat is clogged.
The fact that it will not go past 4K at WOT indicates that the ECM is pulling spark/fuel in order to keep you from blowing up the motor (chipped piston, etc). It is doing this on purpose. Do not do any more WOT runs until you can get these misfires fixed and cat checked out for being clogged or your uncle may be looking for a new longblock.
