I think 5&6 are also the typical failure locations for the HG. 3&4 are for the I4, if the cooling system is neglected, an important variable not yet discussed.

I'm with Ted, you could blow the 3.0 but the cost, plus an inefficient intake system, would make it less attractive than just swapping in a 3.4 - painless (from what I hear) with the 3.0 drivetrain.

I never got good mileage or good service from the 3.0 *IN A 4RUNNER* (an important variable) and it never made enough more power than the 22RE to justify it in my mind - bad mileage AND low power AND a pain to work on, just didn't keep me from wanting to unload my .45 into it on a daily basis. Just my opinion, having owned several examples.