Putting an RB26DETT in without modifying the engine is pretty unrealistic. I'm not saying it can't be done, because anything can be. The RB26DETT is for an AWD system, so the bottom of the engine carries the transfer gears for the front. Installing the AWD on a 240 is simply not worth the work or cost. So, to put an RB26DETT in, you have to modify it by removing the transfer gears and getting the bottom oil sump/pan from an RB25 and put that on to make the RB26 for RWD.
As far as installing the engine and what work is required, this link is to the best and most detailed RB engine install in a 240 that I have ever seen. Nothing needs to be lengthened, the radiator usualy needs to me moved forward of the location it's mounted at. And you see how the mention having to trim off some of the hood brace to fit the engine, if you do use an RB26 with the RB25 bottom and crossmember, it doesn't require the trim because the RB26's intake doesn't come across the top of the engine like the RB20 and 25.
http://www.sportcompactcarweb.com/tech/0309scc_hybrid/