Buy the adjustable one now. Wilwood makes one with a adjustor knob on it. Not sure if your disc drum master cylinder will work on this. I will need to sort that out on my kids car as i am headed that direction with his. As far as brake lines hooking to the xploder rear, you will have to modify them. Xploder rear is metric fittings. I planned on using the stock rear rubber flex brake line for a 74 dart, and drilling and tapping into the xploder axle tube to mount it, then either cutting the xploder metal brake lines and flairing them with SAE ends to thread into the brake block, or bend new axle lines with metric on one end, SAE on the other.
What you can do to get some money back since your wanting to run the ford rear is this, buy the 74 duster disc brake brakeline kit but spec the rear lines for 72 A body 8.75 rear. If they wont let you do this , then spec the rear as 8.25. They will ask you if its manual or disc power brake. I believe the fittings are the same but they give you a little more tubing to thr master with power disc setup. You can always work the tubing a little but it gives you a little extra to work with. Install everything except the rear axle metal line since your using the existing 7.25. Just blow the existing rear axle line out real good, and swap out the wheel cylinders since they cheap, and do a quik n dirty rebuild of the rear brakes on the 7.25 rear. This way you can drive the car safely.
Since those rear brakes are the same from the backing plates out to fit an 8.25, and 8.75, you can strip them off the 7.25, and sell them complete with the backing plates out when you do the xploder swap, minus parking brake cable since you need that for the explorer rear. You can also sell the new unused rear axle lines that came in the kit. Then scrap the 7.25 unless you want to keep it for some reason.
At least thats what i would do. Run it with the 7.25 and rebuild the brakes enough to make it safe. Then enjoy the car with all new brakes, suspension etc, while rounding up all the pieces to do a V8 swap, so it can be a weekend swap. Then run it until you can get your xploder rear and have it ready for a weekend swap as well. Do this in modules so you can enjoy the car before each next big project.