What am I missing ?

[yellow8]

You do yours now while I'm still gathering ideas. That way I can learn from YOUR expensive mistakes instead of making my own.

Talking about costly mistakes. I bought my '75 K5 in Nov. last year. I was told it had a 4" lift with blocks and was sagging about 2". I swapped the blocks with 56" 6" lift springs. It worked perfect leveled it out nice but after driving it for a few months I decided I didn't want it that high. I just got done installing the ez ride 2" springs and like the height and look much better. It lowered the truck just over 3" so I don't think the 4" lift was accurate.
If anyone is looking for some good springs pm me, I need some money.

 

[K85 Octane]

If you end up too high in the back, you can always pull the overload and run the blocks for your offset. They are damn near the same thickness.

 

[CrpMag122]

Still no answer to if I have a full float 14 bolt in my 87 K5 will this affect driveshaft length etc with the shackle flip 4.5 from DIY4X? Or just lift and find out the answer?

I don't know if this helps, since I am running a different setup, but I had to get a new shaft. I have a 4" ORD spring lift with 2.5" shackle flip in the rear (just over 6" total.) I didn't run into so much of a length issue as more of an angle issue. The shaft was too short by a small amount and honestly may have worked if it was not for the angle. I had to end up buying a double cardan slip shaft from Tom Woods. $480 but it sure is a thing of beauty.