Don't know the spring lengths- I can measure my 87 tomorrow.
Trailing arms and coils are a FAR better ride. Converted our 67 to coils with a No Limit long travel kit and it's SO much better, and really hooks a corner.
From my experience the arms are the same, size wise. Find a square and get the PS box and crosslink ('82 and up are different). Some spindles push out less than others. There are some charts kicking around for all this, lolife99 has a good one.
Don't know about the MC- Rokauto I have found to be a good tool for digging that kinda stuff up. I'd do the 3/4 ton caliper upgrade.
this ought keep ya occupied-
https://67-72chevytrucks.com/vboard/...d.php?t=647790