Re: some grunt into 156 v6
All the external dimensions of the V6 24valve engines are the same so fitting it in is not the issue. As I said...putting too much power through the transmission is the issue..they break. Can't put it much plainer...
As for boring..well these are engines with liners, so you can't bore them.You can put piston & liner kits in to make them bigger capacity..but this is seriously expensive..ditto turbo charging, so if money is an issue..just forget it.
If you are looking for a very fast well balanced sports saloon, just stick a 3.0 in. It has usefully more torque than the 2.5, and ,makes a really lovely car without outrageous cost..but don't think its a dirt cheap swap,as you will be well advised to get the cambelt and water pump and tensioner done as well, plus a clutch and radiator. You could be looking at £800 in parts alone...$1400 ish?? plus your engine and labour.
Its perfectly doable, but don't think of it as a small job.