For a truck-arm application, if you just want drop & don't care about ride/drive quality, just buy 4 drop springs & replace hardware as needed.
If you want drop while improving the ride & drive quality, here are things to consider:
1) Just adding the spindles mentioned won't get your desired front drop amount (Western Chassis spindles are 2.5" drop). There are 3" drop spindle options, but those usually increase the probability of interference.
There are 3" drop springs for the front. They are not optimum (when all things are considered) but will work. A combo of the 2 is best. Keep in mind, advertised spring drops are compared to stock heights & can vary the resulting amount of drop.
I know (from reading here) spindles and new springs are the best combo. I was just searching some pictures on here of lowered trucks - there were several 2/4 drops that look like they were the height I'm after. I know the difference between 2/4 and 3/4 or 3/5 doesn't sound like a lot, but it really is when it comes to this. The more I think about my current ride measurements (from center of wheels/hubs), the more I think a 5" rear drop will be too much.
Here's my info for stock ride height: http://67-72chevytrucks.com/vboard/s...d.php?t=746570
2) Even @ 3-4" of drop, I would use rear shock brackets to stand the shocks up @ a better angle vs. stock. In my opinion, any rear spring lowering hurts shock effectiveness by changing it's designed working angle. I would also avoid marketed/kit 'drop' shocks. Shorter shocks don't help ride quality
unless valved properly & they're installed @ good working angles. Typical drop-kit shocks are just shorter shocks that were made for a different application (usually the front of a diff vehicle) but can work on a C10. For the best ride quality, use good quality replacement length shocks (or as close as possible) & tweak the brackets as needed to put the travel where it should be. 'Kit' drop shocks are a typical
added expense where it might not be necessary. What condition are the stock shocks in??
* REAR truck-arm shocks/mounts: If you are mechanically capable & can fab, tweaking the original rear shock mounts for improved travel can be done in a couple of hours for free or new mounts that improve the working angles can be made for pretty cheap.
* FRONT shocks/mounts: Once dropped, taking some measurements will guide what your needs are. You may/may-not need to do anything for the shock mounts/shocks to work as intended (esp. if using spindles for a mild drop).
I'm not sure on the current shock condition - the truck rides extremely smooth! But, I've thought about going ahead and replacing the shocks at the same time, just so I know they have been updated. I work at a manufacturing company with full weld shop and capabilities. If needed, there are a few guys here that could probably help me out. I could probably even design some new brackets and have them made.
3) When lowering a truck-arm C10, the axle will/does shift over. If you don't mind it not tracking straight, it can be driven. However, some trucks might experience rubbing depending on wheel size, back-spacing, & body placement on the frame. Adjustable Panhard bars help correct this & keep the tracking straight.
That's good to know about the panhard bar. I definitely don't want the truck tracking oddly. I'd rather have it driving straight down the road. That's the kind of info I need. 
If it were me? I'd start by seeing what condition my front suspension & shocks are in. That helps kick-off educated decision making.