First off, I ordered some Belltechs and part number says 6400-100



Just wondering if this is the correct part number.

Also, will I need longer shocks once these are installed? I added 2" blocks to an 05 silverado and the stock ones bolted back up but were extremely limited in travel.

 

6400-100 works, haven't fully cycled the suspension yet to see if the shocks will limit the travel or not...but so far so good...3" springtech spindle and shackles work good together

 

Yes that is the right part.
No I don't believe you need new shocks. I never bothered until my old ones wore out that is.

 

YES you will need new longer shocks!
I installed the same shackles on my 2011 FX4 and on the 2" it only raised the rear 1.5" i did'nt think much but the truck rode like crap. i lowered it to one inch spot and it droped only a 1/4 inch! and it rode alot better, why? simple the shocks limited the travel. I have a 1.5" inch block from stock with my FX4 so that might be what makes me ride maxed out, "double lifting" the rear,
Regardless i will be starting a thread looking for longer shocks
Note i also found the front shocks limited after a "1.5" inch crank, turns out i over cranked it to 2" after the shocks released tension and i spaced the fronts out a half inch.
There really is not much play in shock travel from OEM in my experience

Intall the part, (very easy) check measures before and after both sides on level surface (hop up and down on the bed a for a bit before and after to get a "feel" for suspension activity
go from stock to 1 inch to 2 inch measure at the wheel arcs inbetween and check suspension feel at every point,
I bet your gonna need longer shocks with the 2" blocks