They're actually 64"s BTW.
Yes, that hanger is bolted in properly now. I just got excited and had to play around with the jack and the camera.
Looking at rigs in junkyards, it looks like somewhere in the '80s they moved the body mount behind the doors to outboard of the frame rail. This would make it a bit trickier than mine was (also a 1976), but certainly still doable.
Other than rivets being a pain in the ass, it's really easy to do. I am thoroughly impressed with the ride quality and travel that my 900-series trucks have in the rear bone stock, so I'm a huge fan of the 64" springs. Plus, the ones in junkyards are obviously going to be newer and better condition, and as you mentioned, plain easier to get a hold of. Being a foot longer has all kinds of benefits with travel too if you do the trig.
If you want help figuring out where to move anything, I'm happy to help you with the math. It's really simple trigonometry to get your shackle angle correct (you can't necessarily just set it to 35*, there's a lot more variables than that). A scientific or graphing calculator that has trig functions will let you just punch in numbers and receive answers without questioning the "magic" that goes on inside.
You will want at least 6" shackles. Shorter than that will really be limiting the abilities of a spring that long. Plus, the swap will require significantly less work with the longer shackle unless your chopping up the frame/body anyways. You probably won't sit any higher than you are now even with a 2" longer shackle due to the way the shackle will sit. Absolute worst case scenario would be sitting only 1" taller than you are now.
