Javier,
I will have the very problem(s) you are considering. As my car will not be arriving for about a month there is not a lot of specifics I can tell you. However, the video of my car helped me outline the things required.
My unit is a DRB and this setup is different from the GTDs. However, the basics are the same. You will need a new dash ( can probably sell your old one to offset ). You cannot invert the rack and pinion. The steering will be backwards. Turn right and the tires turn left. You should be able to find suitable replacement, either through your local junk yard, such as the mazda 323 etc., or go the aftermarket route. Like those found in Rod and Custom magazine or other restoration mag. Just know your measurements before you order. The pedal assembly can be moved fairly easily, but be up on your welding skills. You may want to change the type of assembly at that time if you don't like the current setup. ( floor to swing ) Especially if you have big feet. The swing setup allows you to space the pedals a little further apart. I can't comment on your tunnel. Not familiar with GTDs But you should be able to fabricate a new tunnel to house all the cables and tubes. If nothing else, it will give you a chance to inspect what's in the tunnel ( rust, leaks, insulation ). Your shifter choice will be dictated by the transaxel you will be using. Then your tunnel mods will be dictated by that. Figure these expenses into the price of the car you are looking at. Then decide if the hassels are worth the difference. It all comes down to how much are you willing to do, or put up with, to get what you want.
Bill