Where did you apply the heat - the bolt itself, or around the bottom of the female threads in the block? You definitely want to get the aluminum to expand around the steel bolt, then do as Recluse suggests above. Spray the bottom of the welded nut with penetrant, then use a punch and hammer to hit straight down on the bolt shank several times. Then, start rocking the welded nut back and forth.This bastard bolt…
tried heating with a propane torch and turning with a vise grip.
As tight as yours is, you may be headed to a TimeSert installation too.