As previously stated, unless you are a street racer with a lot of room to run, the 3-speed is a waste of money.
For bashing, you lose your speed and rarely get to the topside of 2nd gear.
For offroad racing, you rarely if ever see enough straight away fo pick up the topside of 2nd gear let alond bust into 3rd.
If you want to spend your money on something worthwhile...look into another area of your truck that needs upgrading.
And to answer your original question...YES.
The .26 engine is a .21 bored out to make a .26 displacement. As Christian mentioned, the outside is roughly the same, it's the guts that are different.