I have a 1998 Chevrolet 6.5 liter Turbo Diesel. When I start it or even after an hour of running it blows white smoke. I also have a leak in the coolant, as i have to refill it occasionally. Now before you say its the HG i replaced both the HG's and both heads. The truck runs great, doesn't leak any fluid, and the oil is clean. Could there be another place that the coolant could be dumping directly into the exhaust.

 

Cracked block,#8 cylinder quite likely.

Did you use new head bolts or arp studs when you did the gaskets/heads?

The stock bolts are 1 time use only,throw away TTY bolts.

The only place the exhaust is fitted to anything near the engine is at the exhaust ports,coolant must come through the cylinders or chambers to make it to the exhaust.
The intake is totally dry so no way for coolant to migrate that way.