Have you figured out how to lower the truck? Buddy of mine had a '74 short wide with a 318. We swapped in a junkyard 5.9 magnum. Re used his 727, headers, etc. Used a Dakota airbox, harness, pcm, and such. Was a pretty cool low buck truck.
He bought axle perches from a trailer parts store. We cut the old perches off and put the axle above the springs, used the new perches and u bolts, set the pinion angle and welded it up.
Pulled the lower control arms off of a van and cut the trick springs to find the correct height. We cut them until we found the height he liked and then measured the installed spring height. He called and ordered a set of heavier duty springs with the correct height. I think he got them from eibach.
The van uses a bigger control arm bolt compared to the truck if I remember correctly. We ended up drilling the bolt holes bigger on his truck and reused the van control arms and bolts. I also remember reading somewhere the 3/4 or 1 ton trucks might also use the van style control arms. The spring pockets are deeper in them.