Um for one, unless someone went through way too much trouble to swap in a 2.6L the B2200s have a 2.2L, the B2000 a 2.0L and B2600s have the 2.6L and so forth.
If the truck does in fact have a 2.6L in it now then you should be able to take the bell housing off the B2000 and put it on the 2600 tranny. After that it will bolt right in. This will involve removing the tranny.
And no they arn't completly different transmissions, they have different ratios, a different bell housing and a different tail housing, everything else is pretty much the same. I modified the transmission in my truck with parts from an 83 RX-7 Tranny.
Transmission Modification Page