WD40 is a great product for rust removal---the "WD" stands for "water dispersal." But it is absolutely NOT a penetrating oil, despite some of the company's marketing claims. I really like WD40 for its intended use, and there are always 10+ cans sitting around my shop at any given time, but it should really be used for light rust removal only.Tried the puller method after soaking it in WD - no chance.
There are a number of commercially available products that are actually penetrating oils (and some work pretty well), but a number of years ago I got turned onto a 50/50 mix of ATF and acetone as an alternative---I have used it for a few years in working on 40-50-year old Porsches and BMWs with great success.
My recent junkyard GTV6 project had spent its entire life in the midwest and on the east coast and all of the fasteners underneath were rusted, which makes spoiled west coast mechanics like myself cringe! However, I made a point to liberally apply the ATF/acetone to anything that I was planning on attacking several days beforehand. I have had a lot of the car apart in the 4-5 months that I have owned it, and the only casualties so far have been a single rounded-off rear brake rotor bolt and one snapped rear brake hard line (I cracked loose every other brake and clutch hard line fitting with no issues!).
Last weekend I tackled the driveshaft and the rear engine mount and DeDion pivot mounts. While it was a ***** to partially remove the bellhousing with the engine in the car to allow enough access, the rusted-in rear engine mount actually pressed out quite easily. And I was able to press out the original DeDion pivot bushing BY THE CENTER PORTION and with the rear axle still hanging in the car by using a C-clamp and a standoff---the homebrew penetrating oil obviously did its job.
Now, your results may vary, but it is cheap and easy to try before resorting to more extreme methods . . . .