Catching up on some projects, and I love these trucks.
Your best bet for cost and availability is a 05+ super duty front. On that note, you will either have to retube it for pass drop or swap t case for driver's drop. For even more strength, a 1550 joint and shafts are available, and they are factory parts so they don't carry the same price as RCVs or the like. I think I saw them for less than $400 (each possibly, but could have been together, ECGS has them).
Other possibility is a 99-04 super duty D60, it's not as stout as the 05+, and do your homework as many are D50. Leaf sprung, but I think they are wider perches than square body trucks, and still driver side drop.
A GM D60 will almost require a rebuild, and they are $1500+, at least here (NC), if you can find them. Almost better off building a 609 for the strength and comparable pricing, just requiring quite a bit more work.
Love a LS square body. Like it's designed to go in!