Hello out there. I have an 06 500 Limited that I love and imagine keeping forever. I know that I will one day need to replace the motor or rebuild it, and I would like to have a bit more power. I've noticed that my grandparents Mercury Sable has a 3.5 liter, and thought it would be a nice substitute. Has anyone heard of anyone swapping these two?

 

The 06-07 Ford Five Hundred/Mercury Montego were replaced by the 08-09 Ford Taurus/Mercury Sable. Even though the 06-07's came with the 3.0 Duratec engine, the bays are the same and will fit the 3.5 Cyclone engine. You most likely will need the tranny and computer from a 3.5 donor vehicle as the trannies used with the 3.0DOHC were dropped when the 3.5 came into service.

 

Not to poo poo your idea, but the days of just taking out one engine and inserting another (not an exact replacement) are gone!
In addition to Use Common Sense's info, it would be wise to look over the electrical diagram for both vehicles and compare the differences which no doubt are there.
If you have unlimited time and funds it can probably be done. Don't know if you really want to get that deep into a possible swap. Not trying to be funny, but might be cheaper in the long run to just use premium fuel instead of regular grade. I would expect you could see some improvement in performance, but wouldn't guarantee it.