Cylinder Head

[zmster2033]

Having some issues with my ISX burning coolant. EGR cooler and valve have been replaced along with pressure testing the entire cooling system at this point. Truck has white smoke out of the exhaust, and smells sweet. No performance issues or anything of significance other than that. Going through a gallon of coolant every 600-700 miles or so at this point. Thinking we are looking at the head. Planning on taking the exhaust manifold off to see if it is wet/signs of coolant. Is there anyway to determine if coming from head/block? Called the dealer (KW) and they provided cylinder head part # 5708760RXCUM with a price of ~$7,800 + tax. Other ISX heads (different part #) look to be significantly cheaper when searching online. What would cause this difference in price? This is a 400ST engine CPL 8520 per engine tag. I found a listing on ebay for a whole engine for $27,000 obo which doesn't seem all that bad considering. ECM is showing 910,000 on the clock, I believe this is the original ECM. Any suggestions on how to proceed? Is there a better CPL to look into getting, and is it compatible with existing harnesses we would already have? This is my only ISX in the fleet, so still trying to learn the in's and outs. Seems like cylinder heads are common on these things though. Appreciate the advice in advance.