Subaru forester
3 Answers
The Forester never came with a 6 and you would be on your own. This would be a very expensive swap if you manage to shoehorn the engine in. Buy a Forester XT if you want a faster car.
I know the forester didn't come with a 6, from what I've read in a couple articles is that the 6 is a stronger, meaning more durable longer lasting engine. Speed also not so much a factor but heat would be if the 6 was too tight a fit, if it can fit at all without much trimming.
Take some measurements if you want to pursue your fantasy. You might very well be the first to do this swap.