I think after wearing out my tape measure and getting in and out of my truck, and looking at @wetskier2000 and @JOYFLEA builds, my best option is to get the torch and cut a big hole in the back of the cab.
Not because of the problem you are having, but because I had very little room side to side, I made my own seats. I found 2" of foam to be plenty on a seat back. If you stand the steering column fairly straight up and down, then the seat back can also be closer to straight up and down. I think you could save almost 6" between seat back and firewall over what you have mocked up now. These seat backs are 3" foam, 1/2" plywood, 1/8" aluminum and a 2" lip (where all the rivets are) that could be left off. I have more angle than most would need because I have bad discs in my neck and must drive laid back with my head tipped forward. BTW, bucking rivets is a lot easier than it looks. An air hammer, a special peen, an air regulator and a chunk of steel does it.
I think the available room for seats goes to your desired seat back position. I like to sit pretty much up straight so I could have survived in my cab without the sleeper and obviously sans the in cab fuel ta k. Wife leans back and would not have adequate seating.
Mocking up the sleeper. Call it the cardboard camper for now. Will have 6 inch radius corners. Door on each side. Step we’ll for door and battery box and storage below sleeper.
Yikes. I picked up steel for the Sleeper Friday and nearly*****ped my pants. Cost was triple what it was last summer, so the guy tells me when we are loading the trailer. Oh well, like they say. “Ya wanna play, ya gotta pay”. At least I will have somewhere to sleep. Also found two more nice leather seats for the mother in law compartment. Sleeper seats tumble forward so that the bed will fold out over them. Hard to explain, but will post as we go.
Building a custom shifter. Will use cables from front to back to move existing shifter stub. Looks good on paper. Guess we will see how it does in real life.
Almost bought these at a swap meet a couple weeks ago. Then got worried if I ever broke one, where would I get another?
Was rummaging around a wrecker and like a magpie I saw this shiny thing. $300. Have no use for it other than cool wall art.
I heard at a car show on the weekend you’ve been working on your truck, pictures? Also Charles would like some contact info for you.
Finally back on the job. Got another new knee in July so now I have two good wheels under me. My daughter moved back from Vegas after 14 years away. So awesome to have her and the family back in town. She kept me busy before and after surgery doing a couple projects at their new home. We had our first snow so it’s time to lock myself in the shop and get going. Granddaughter loves grampas “big green truck”