Can I reuse my pinto brake backplates and drums on the mustang II rear end housing? Is it just a matter of swapping the pinto brake setup mustang II rear end and axle. ( I know axles have to come out to put backplates on .)

74/80 Pinto backing plates / drums are the same as any Mustang II. This was done so Ford could use up what they (6 3/4") already had in stock. The Mustang II 8" rear end is a odd one. Center section (gears) are all the same in any 8" rear end. Some but not all of the axles will fit with a little work (shorting). The axle bearings are a one of a kind to only a Mustang II. The axle bearing retainer is different from any other 8" rear end and is MUCH shorter than any other 8". Put the long one on the axle and the axle will not go all the way into the housing.  The Mustang II rear end is a direct replacement in any Pinto. Had them in my 72 and two 80's Pintos. Drive shafts for a 6 3/4" will work just fine with a 8". Been there and done that.