I added a '03 OEM double cab fold flat to my '13 Tradesman crew cab. Went in just fine, took some nuts, washers and bolts but looks and behaves like factory. It's just a little short to the front seats when folded down (double cab length fold flat). Came off my father-in-law's '03 3500 after he totaled it.
One could also do the mod that allows the back seat seatback to fold forward at the same time. i didn't do it because it required cutting the seatback hook and adding a stack of janky washers, if I remember correctly.