He was having a problem -- that is why he changed the thermostats in the first place -- Blown headgaskets are a common problem on LB7 engines and I am sure they were already blown before he did the thermostats.
Replacing the gaskets is a lot of work, but can be done by someone with average experience. Remember that there are several different gaskets and are identified by a tab at the rear of the engine-- be sure you use the correct ones when you replace them.
Good Luck 
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Chevy 04.5 LLY, 265s, shell, XDR, MikeL 6spd, TransGo, deep pan, blocked, fingered, de-badged, PVC, 65 Gallon Aero tank, PPE, LBZ- mod, Pre-fuel filter, tow mirrors, BU cam, gauges, GPS, Sirius, bags |