need some pointers

[lt1 chevelle]

i just got a new qa1 rear coil-over system for my 65chevelle and i will be installing it this weekend . this is my first marjor time doing a suspention mod that requiers welding and was woundering if any one had any pointers or helpfull info.also it says to measuer the ride hight but i want to lower it 3in. so say if the stock hight was 22in when i go to set everying up can i mock it up as if the hight was 19 that way it will lower it the 3in? or is there a diff way to do that?
thanks chris