In most cases, you will be fine with a traditional drop spindle, non-modular. What type of suspension are you running, bag or static drop? What wheel sizes? Some guys want the wheels pulled in to allow for more turning ability at a lower ride height. This is usually the case with air suspension, or an extremely low static drop.
If your truck has a 4"/5" or something similar, you will be Ok with the non-modular spindle. With the modular spindle, and using stock brakes, you will need an adapter bracket for the caliper mounting. The modular spindles don't come with caliper mounting provisions, and are usually used when running a Baer or Wilwood brake kit. The traditional style drop spindles will have caliper mounting "ears" on them, like your stock spindle.
The traditional style drop spindle won't widen your track width at all, since your truck is already a disc brake truck with a disc brake spindle.
If you don't plan to run a Baer or Wilwood big brake kit, then I would suggest go with the traditional style like this:
http://www.classicperform.com/Store/...ruck/CP4S1.htm
Or, go with the McGaughy's here:
http://www.azproperformance.com/McGa...173-63176.html
We can get both spindles for you, just let me know.