set up rearend

hay guys i have a spare 8 3/4 rear that im thinking of setting up for my duster .right now i have 1 in it but im useing shim to get the pinion to where i want.. but i think i would be better to just take my spare and cut off the perch and weld in some new 1s at the 5 or 7 degress that i want and not have to use the shim....my ?? is do i just place this on a brace at 0 or whatever its at when leval and cut and add the needed degree, or do i need to put a center in and get the amount from the front of the center and go from there.....:toothy10: