back spacing and sizes

I do have a spare A Body 8 3/4 I picked up at You Pull It. Was 80 bucks with a power lock 3rd member.
And a Doctor Diff kit. So going to move spring's But they did fit but are tight.
think I need the room since these are 15x8 Ram Charger 4.25 back space and would like to use them with 275-60-15
My other wheels are 4.5 back space



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