Hi there Nunzio,
The front end is pretty much a bolt-up job. There is a tech-sheet on how to do it here:
http://www.fefcholden.org.au/techinfo/tech002.html Likewise the rear-end is a straight-forward swap.
A red-motor is almost a bolt-up job. The mounts on the HR front end need to be modified (see the article for more), and because the red is a bit longer than a grey, there's very little clearance for the mechanical accelerator linkage as well as the standard fan. Usually one of these has to go, or the radiator has to be moved forward (as was done in my car).
However, there is a lot of work in my floor, although that's not necessarily the way you have to do it. We used (thanks Dad, thanks Charlie) the floor out of a HK (which also donated the 'glide and the T-bar and console), and trimmed it to size, cutting out virtually all of the FE's original floor. Only the central hump support remains. It's very strong, but there are easier ways of doing it. You can cut a H-shape in the floor and open it up and weld in a new top to the tunnel. You are best off to speak to an engineer (who will usually give you some advice before you start for nothing). I recommend John Varetimidis of Lohning International (they're based in Mortdale). John is the engineer who does the technical write-ups in Street Machine's "Expression Session" articles.
As mentioned in passing above, I use a T-bar shifter. With a left-handed gearbox, getting the column connected up was just a nightmare I wasn't even going to consider. In any case, I was replacing a previously installed 4-speed, so already had bucket seats and a big hole in the floor. That hole was almost a lot bigger when the main shaft in the 4-speed snapped at 60kays!
Not to mention the effect on my shorts :-/
cheers
RET