I may be wrong but my guess is that the motor and trani weigh well over 450 lbs which is max rating for HF jack. I also would worry about the lateral strength of this jack. Could it twist under max load and collapse?
Nick has a well proven solution. Then you have guys like me..... See post #31 through 36 in
http://forums.pelicanparts.com/porsche-911-technical-forum/748639-evolution-carrera-rst-2.html
Note that the motor sits on a wooden platform to minimize the chance to dent the heat exchangers.
The jack that I use I got from Costco. 2 ton or so with a wide seat and 20 inch max lift, I think.
Hope this helps. Good luck.
Cheers....