What Else?
 

So I'm replacing the Head Gasket and the Heads what else should i look at replacing since I'll have it broke down? Its a 96 P38 with 172,000. I'm doing the head gasket b/c my no 6 cyl is missing, when i pulled the valve covers I saw a ton of sludge, which leads me to believe that miss is due too carbon on the exhaust valve. Other than the miss on the 6 cyl it runs fine.

Are you sure you need to do that? Could just be a coil pack, spark plug, or lead. Btw, a little sludge is normal. My parts under the valve cover are tarnished but not covered in that oil pudding crap. There is also the possibility that you bent a push rod.

If you adamant about taking off your heads, I would take the valve covers and heads and let them soak in some carb and choke cleaner. that will really clean things out ;). Get your heads machined too. While you are waiting take the oil pan off and clean out the sludge in the crank case too.

I had tons of sludge. I replaced the Spark plug wires, spark plugs, and checked the coil for spark. I can hear a miss plus I'm getting codes 1314 1316 306 300 and the right bank knock sensor sometimes. I tried seafoam induction and all the other snake oils. I recently bought this as a project and thats what its been. I have all maint records from 1 to 102,000 miles but nothing from 102000 to now. I'm betting it had irregular oil changes and ran on 87. I was thinking of soaking the injectors in seafoam.

YUCK! I guess yours does need to be rebuilt a little. 87! though WOW!