Not sure what spring anchor is or if you still need it? (assuming Blazer rear end means you are going open drive shaft) Also I believe your rear end will "rotate" rearward when weight is applied. The shackles will possibly swing, about the hangers, to the rear a little under load of the vehicle sitting on it's tires. (not positive about that). The wheel base is what you are looking to measure, as stated, under load, because you did not bob and mark. (Those are not two guys.) If the perch/spring locating pin is NOT in the center of eye to eye, you CAN put them on backwards.
