Could be the head gasket. Did the head rebuilder check the head for cracks?
How about the thicker head gasket? or is it a stacked set of gaskets?
You could do a leak down test to figure out which cylinder it'd coming from
Also, on a Diesel, you don't worry about lowering the compression for running
higher turbo boost, the more the better, some tractor pulling diesels run compound
turbocharger systems boosting the engine by as much as 100 PSI
this also
adds quite a bit of power as well, the advantage of too much air in a Diesel engine
is that the exhaust gas temps go down, and burning all of the fuel instead of
leaving black smoke clouds