I am in the planning stage on mine. The fact is that I have 3 young boys and a girl due in a few weeks and I want something unique to do some real wheeling with and take the family. I currently have a 12" stretched Bronco that I can fit everyone in while using a second rear seat installed but it is pretty cramped in there and I dont want the rear seat all the way to the tailgate for safety reasons. I did roll with the fam in it at Big Bear BB this year and it worked but getting to the middle and rear is tough.
My new plans are to take the 76 I have in the garage that is halfway to bare metal and cut the tub horizontally where the floor rises on the driver side, slide the whole rear section back 26" and add a middle striker striker pillar that will be reinforced with the cage at the B-pillar that will go to the floor and subsequently to the frame. I am doing 26" as thats 2/3 of a door and is right at the vent window vertical (FWIW the JK rear doors are 29" and curve down so I think ingress will be fine). Cutting it down here will allow for all the internals to be reused even the glass, I will just need to add a section of window frame to the front where the vent was. The rear doors will be suicided, I will just modify the rear striker to be a post instead.
I am going to try and lower the floor the whole way back until you get to the rear bed floor area where the 3rd seat will be. Going to try an use a full set of seats out of something like a Honda Pilot or something kinda midsized that has a fold down middle row and a narrower third row. Possibly an explorer middle and a Tahoe 3rd row if I need to miss match.
By doing it this way a hard top can be easily stretched to fit as can a soft top since they will each just need 26" in the middle. The only section that I am still working on in my brain is potentially a needed removable upper pillar above the middle pillar for when the top is on to fill a small gap there and something for the weatherstripping to mount to.
The 26" extension will give a 118" WB which is right about the same as the JK 4 dr so it should still be wheelable.
I will be using a 97 explorer with efi and rjm harness which is all in the garage, trans I am contemplating 3 options, either a 4r70w and np205 which I have or possibly going with a divorced setup to make it so the rear drive shaft isnt 5 feet long and would allow for a good belly pan to be used (as I'll need it). Also contemplating a zf/203/205 setup like I have in mine right now as I am really happy with it and the gear ranges...and I love a manual trans. Currently looking for a D60 front and 14blt rear as I want to go straight to 1 tons on it.
Anyways, those are my thoughts as of now, hoping to start working on the stretch next month.
I attached a modified 4 door to show about what the dimensions will be with the 26" door. The second pic is my photoshop version if I modified the rear quarter section to do the 4dr. Doing it this way would take a LOT more work though and so I scrapped it. It does look good though but would only be a 4 seater still. You can see the half cab version above ^