I've pondered doing something like this, but you have a LOT more experience than I, so this is just a wacky suggestion...
Mock up the old cab so you can see if you can fit comfortably (seat, mock column and wheel, blocks for pedals). This will answer whether extending is a luxury or a necessity. I bought a std. cab Dakota figuring it wasn't THAT tight, but found I just wasn't comfortable and if the seat moved back couple more inches it would be a huge difference.

I wondered if cutting out the early cab floor and firewall, then cutting the Dakota donor windshield, roof and rear of cab, leaving essentially the firewall and floor and marrying the two would allow for the modern mechanical bits to stay in position easier. I can see even before you mentioned the measurements that the firewall would need to be narrower, but some careful measuring and maybe moving some things might be an answer. The big things would be where the steering wheel and pedals fall in relation to the door and where you sit. Moving wiring and HVAC is simple compared to making the steering column move 2-3" toward the engine!