Just know, that arm is a taper fit, penetrating oil won't due much to a taper fit. Put the puller on and tighten the bolt till it about breaks, then hit the head of the puller bolt really hard with a hammer, bigger the better. Have the nut on the shaft so when it breaks free it doesn't hit you or something else when comes off.
If you go the heat route, there is a seal right at the housing that will likely get damaged, it's easy enough to change once the arm is off, and should be changed anyway, heat or not.
Us a puller made for idler arms, not a long arm gear puller.
