Originally posted by Paul Herioux:
The offset of the wheel cylinders is different between the 12x2 1/2 and 12x3 brakes. You will need to change wheel cylinders to go to the wider shoes.
Your post didn't say how you truck is configured, if you have a van box or other build up, or if you do a lot of towing on a regular basis this is an worthwhile uprade. If your running empty most off the time the added braking bias to the rear will make your RWAL system have a little overactive and possibly turn on the light.
Good luck, hope this helps.