you cant really be 100% accurate with the spring perch angle, without putting it in the car at ride hight with the driveshaft installed and the rear end centered in the wheel wells. Tack the perches in place, remove and then weld!
I can bring my 8 3/4 housing and let him match it. When I moved the e body perches to a body width I changed it 2° just stock angle and it seemed to work fine.