Please, for the sake of your truck, DO NOT build a square drive shaft! You will not be happy with it at all if you drive it on the street in 4 wheel drive. They are are sloppy and noisy, any you can't possibly weld them together straight.
My solution: get the extended drive shaft kit because you will need it. SAS kits have too much travel for the standard toyota 5" splines. Use your '85 double cardan joint.
Grind the weld on the tube off with a grinder. Using a cutoff wheel, cut a ring into the tube about 3/8 of an inch back from the cardan joint, only go as deep as the driveshaft is thick. Then split the tube longways with a cutoff wheel and peel the tube off. You have to split the tube because it is a press fit. It's no big deal if you go a little deep, the flange can take it.
Go to the junkyard and get a driveshaft out of a 2 wheel drive toyota because they are long and you can use the tube. Measure how long your shaft needs to be. Press or hammer the new long spline kit into one end of the new tube, and your double cardan on the other, making shure the U-joint ears are in phase and there should be about 2 to 2 1/2 inches of spline showing. You can mount the driveshaft in a vise and use a level on the U-joint ears to get them very close to in phase.
bolt it up in the truck unwelded, hubs unlocked, and you can spin the driveshaft by hand and check for wobble. I use a dial indicator to true them up, but you could eyeball it in a pinch. tap the high spots with a hammer to take them out. Throw a few tacks in it, then remove it from the truck and fully weld it.
Sounds complicated, but it isn't. I've built three front driveshafts this way, and run mine in the snow every winter.
