For what it's worth...while the bench seat itself from an extended-cab truck is certainly different from a crew cab or a regular cab truck, I'm having trouble understanding how the seat frame or the floor pan's different. My extended-cab truck originally had a bench seat in it, and I replaced it in '06 with a seat assembly that came out of a crew cab truck. The seat assembly was from a Centurion conversion, so it came with a frame that bolted to the stock bench seat mounting points in the floor pan, and the frame has mounting points for two bucket seats and a center console.
So, based on that, if the extended-cab truck has a bench seat, any bench seat from a crew cab or a regular cab truck should bolt right in.
However, as I understand it, any truck with bucket seats from the factory has different mount points...so, if you want to swap factory buckets into a truck that had a bench seat, you have to drill and reinforce new holes. But, again, the difference in floor pans is in regards to trucks that had bench seats vs bucket seats from the factory...the cab style (regular, extended, or crew) doesn't make a difference in this case.
FWIW...and I would like to someday figure out a safe way to raise my driver's seat a couple of inches...