I have 2 old Blackburn racks and have used them on a number of different bikes. Each time I would make some bends to customize to the new frame, both to spread/compress or level the rack. I have never had any issues at all. The rack I am currently using on my commuting bike has ~ 30 years use. 5-7 mm is nothing - go for it. Remember - slow and steady pressure is what you are after.

Good luck