I've had my fair share of the "bhg syndrome" as most of the supra guys call it! One of the things that most folks forget to do after the head gasket replacement is to "burp" the cooling system.
I can't tell if you have a 22RE or a 3VZE, as you seem to talk about the crossover pipe, which the 4 cylinder never had, but you only seem to say head gaskeT (not plural) and header(again, not plural), so I can't really say how to burp the v6 engine.
If it is a 22re, then get the front of the truck up in the air as high as you can, let it warm up with the rad cap off, turn the heater to full heat and wait for all the air to come out of the system.
A steep driveway AND ramps would probably suffice. If after you do this, drive around and flip from cold to hot on the heater, if you hear slushing or gurgling noises, you have not gotten all the air out of the system, do it again till you never ever hear that sound again!
That little bit of air stuck in the system does NOT always come out, and if there's enough of it, the coolant can cavitate inside the engine and not get to the radiator, overheating the engine and popping another head gasket! I blew a perfectly good head gasket once a long time ago just because of not doing this to my supra, blew the damned gasket within 2 hours of replacing it!
Do you know what caused the last one to blow? Overheat or lean condition? If lean condition, I'd get the injectors serviced too while you have it all apart. Doesn't take long to send them in for a full clean and balance/calibration. I have used RC injectors several times on different toyotas of mine, and always had great results. The local shop in Portland I used once was more money and didn't seem to be any better than RC, and they took almost as long to get 'em done!
