It's all about wheel back spacing when using 15" wheels with drop spindles.
Usually a 3.5" bs is safe.
But I've installed up to a 4" bs with a 3" Belltech drop spindle.
There are many threads about trimming the lower a-arm at the balljoint area of the arm.
http://67-72chevytrucks.com/vboard/s...d.php?t=416186
Some brands also require trimming some of the excess casting on the arm that connects to the outer tie rod.