Doug Hackney cracked two Fuso frames in half, IIRC. Referencing another thread in this category, air ride seats are not a substitute for proper truck suspension and fitting such, to allow one to travel 'comfortably' is simply not a viable option, IMO. Undertaking something so monumental as a cab extension, makes no sense to me, when those resources could be expended fitting nearly any modern comfortable automotive seat, along with proper chassis suspension design and/or tuning.
OEM torsion springs are designed to lift the cab that is there, extending the rear, similar to an FRR or FTR, would significantly increase the load on the forward mounts and the torsion spring when servicing the cab. Additionally, the intake plumbing and locking mechanism would require a complete redesign, including the rear mount(s).
Suggestion: Remove both seats that are currently in your truck cab and see if you can physically fit in the cab. Use blocks of wood, pillows, sofa cushions, a lawn chair or whatever you have to determine a comfortable seating position. A good friend of mine is 6"6" and #270 and can sit comfortably in a crew cab NPR as well as a regular cab FE140. So, there should be plenty of space unless you are in line to be a point guard for your local NBA team.