A "regular" driveshaft, one with a single u-joint at each end from a 1984+ 60 Series - isn't a bad idea on a shackle reversal. If you go with the double cardan, then the front diff flange needs to be pointed up at the t-case flange - likely requiring an expensive/labor intensive cut & turn on the front axle housing. Or just run what you have, shim the front to get the t-case and diff flanges parallel, and feed it u-joints as required.
Either type of driveshaft will need lengthened by a reputable local driveshaft shop to accommodate a shackle reversal, or you risk the slip joint coming apart and either half of the driveshaft whipsawing through the oil pan. That would make for a bad day followed by a strenuous couple weeks/months putting a new motor in after the old one locked up due to no oil.
I would 100% support the transmission/transfer case, remove the crossmember, clean the rust, weld in some extra support, spray paint, and reinstall with fresh hardware that's still available straight from the dealership. That thing looks a little iffy. I'd do a piece sistered to the bottom, some gussets, and definitely weld along the front lip directly below the driveshaft where it looks like the crossmember might be cracking/splitting. More = better.