Had the head gasket failed previously? If so, there's probably a lot of excess moisture collected in the muffler lining, and this needs considerable time and heat to burn off. Been there, done that. If the previous headgasket did not fail, well, you;re probably right, but you should pressure test the cooling system to be sure.

-Jeff

Jeff Moskovitz
1986 turbo 4X4 truck
1998 2.7L/M5 4X4 4Runner

Jeff's 22R-TE page

22RTE Turbo Truck Forum (advertising free)