That brown sludge in the radiator might be from mixing red and green coolants. Some weird chemical reaction goes on that leads to the radiator's quick demise. Shortly after I changed from green to orange coolant (prestone long life), I flushed the block, but forgot to crank the heater, so some residual green remained in the heater core. Soon thereafter, I'd hear gurgling sounds (like you hear from water running down the sewer pipes in a 2-story house) after starting up the engine and starting from a dead stop. Radiator was 60% clogged, coolant looked brown.

One possible scenario is, the prev. owner mixed coolants, that clogged the radiator, then that caused the engine to overheat, blowing the head gaskset. After all, the 150hp V6 on the '90-95 4Runners is infamous for blowing. Apparently, the '88 V6's (first year introduced) have good (ie, contains asbestos) HGs. No problems w/ the 22r engines either, but the 1FZ-FE I-6's in the LC 80s are also known to blow gaskets in cyl. #6, and in most cases, it's from an improperly maintained cooling system. My guess is the HG blew.