There's probably a bunch of ways to go about this, john(Cryptonomad) whose thread is posted in comment 2 did a fantastic job, he incorporated an incredible amount of 5th gen tech into his car, and fortunately for us he made detailed posts along the way.
Mine is a halfway version where I mixed the 5th and 1st gen tech together so as to also take advantage of aftermarket parts available. I had Aaron at ambition road tackle this fabrication for me and I think he nailed it.
We removed the trans tunnel and directly installed the 5th gen tunnel in it's entirety to my 68. we also then had to widen the floorpan by notching the running boards to allow for the wider seats to sit as they should next to the larger transmission tunnel. This also allowed for new seat mounts so the seats could sit as low as possible.
Trans tunnel and smooth firewall
running boards
Dash shortnening
mockup of the dash and console
mockup with the seats
This is post raptor liner, with the dash frame removed. we had it installed with bolts so it can all be removed.
Here is a picture with the framing reinstalled and the 5th gen steering column mounted.
Here are the 68 pedals and original pedal mount bolted to the firewall and came so close to the 5th gen frame that all that was needed was a couple 1/2" spacers.
Ambition road made these awesome closeouts for the ends of the dash, the plastic pieces couldn't be used.
mock up with carpet
I plan to run an LS3/T56 with a PSI harness and the body harness from AAW. so anything from the 5th gen is merely cosmetic. which brought on these following mods:-
steering wheel button delete
double din trim
center console window switches
This is a brief version of what we've achieved so far but I'm happy to elaborate further