Waite. you want to change your thermostat.. 1990, eh are they in a different spot?
am I missing something? has me really confused. what does a thermostat have to do with headgasket or cams.? im sure hes talking about something different.
try this on for size see if it helps.
http://www.cygnusx1.net/Supra/Librar...Section=CO&P=8
and this link has some pictures and descriptions of what is on the thermostat housing
http://www.supramania.com/forums/showthread.php?t=70459
be sure to drain the coolant first. at least almost empty if you don't mind having some antifreeze spill out the ground.