Thats interesting that your 85 does not but the 89 does. Makes you wonder if there was a change over point,,,,maybe after a certain model year it was incorporated into the trans or something. Guess it makes sense since there were quite a few people on here that mentioned their trans not shifting into 4th at full throttle,,,may have had something to do with what year the car was? I know my 84 full size Blazer does not, but my 92 Typhoon did. That may have something to do with the type of vehicle the trans was destined to be installed in also.
A good friend of mine just purchased a new 700r4 from GM,,over the counter,,,for his 86 impala,,,he ordered a special part number trans that was meant for cop cars of that vintage,,,according to some GM specs there are some heavy duty parts in this trans over the standard run of the mill 700. Anyway,,,it shifts into 4th at WOT as well, probably part of the police package or something. Interesting stuff,,,Larry.