406 engine into 306?

[Midnightclub]

How hard depends entirely on your skill really or how much money you have to blow.. you could get some adapter plates made up and then fit pretty much anything you want.. size is the only thing you'll have to be careful of as to whether a turbo will physically fit in the gap down the back.. other option is to gearbox mount the turbo and get a custom exhaust manifold built.. again, expensive but with that you could literally swap around turbos as and when you felt like because it's in a much nicer location..