Toyota v6's including the first few yrs of 3.4 had HG issues up until the late '90's. From what I could get out of the Service manager at my local dealer was that HG issue was do to Toyota designing the engines w/asbestos gaskets, then being forced to use non-asbestos gaskets by the Fed.s, and then failing to come up with correct torque values for the non-asbestos gaskets. This was also an issue with the 7m series of engines. Repeat HG issues were either from poor/ignorant repair work, either not inspecting the head or re-using the torque-to-yield head bolts, and not following the exact procedure Toyota set out in '98. If you did a quality rebuild and went ahead and used new TTY bolts, the HG issue was fixed, of course you still have the issue w/burnt exhaust valves, and leaky valve covers, both of which are also easily fixed. I am not sure which helps more on the burnt valve issue, Headers(negating the poor exhaust design), or setting(shimming) the exhaust valves one # lower than is being measured, I did both to mine in '04 and haven't had anymore issues w/burnt exhaust valves, while I had 2 w/in 5yrs before.
If you are looking to buy a good condition running 3.0 rig for going market value, make sure hat the HG recall was done through Toyota, and not a shop that may have cut corners. All you have to do is call a Toyota Service department and ask if the HG recall was done to the VIN number you are looking at buying, if yes, then you should be fine, depending on what the rest of the service history of the vehicle has been like. Otherwise look for one w/a bad HG, or burnt valves, shouldn't cost you more than $1500 for one in "Good" condition, and repair the engine yourself.
