First off...Nice truck..Really! Got a lil rust repair needed but looks good otherwise.
As for what it looks like has been done suspension wise:
You may already be monoleafed in the rear. Look at your rear leaves, count how many are there, and see if there's one thicker one yet shorter on the bottom of the leaf stack. If there's only 1 long one and one thick really short one under there then you're monoleafed. If you install the 4" blocks between the leaves and your axles, you'll be sitting all the way frame on axle lol. Only way to prevent that is 1. install another leaf, or 2. buy some airshocks and keep them aired up enough to keep your frame and axle from hitting.
Your front has definitely been lowered, most likely via twisting the front torsions.
Basically, imo, you're about as low as you can go without baggin to have some decent ride left. You might could grind off the bumpstop bracket, and/or flip your ball joints and achieve a lil more drop and lil more ride out of it...I'm asking about that in another thread to possible try on mine
Here's a pic of my blocks, leaves (monoleafed), frame and axle clearance WITH 3" blocks and airshocks.
Without my rear shocks aired up some I would have been within probably 1/2" frame to axle.
And here's how low I was:
At this height, bumps are a slight problem...the bigger the worse they'd be . I got tired of it myself and raised it back up...for the 2nd time, but am now considering going back down IF I can get more or a ride via hackin off the bumpstop brackets and flipping the balljoints.
Good luck! You gotta good start.