I agree with the last two responses. English arrowsmiths in the past used weight to judge shafts because they didn't have spine testers. You have one and pay much more attention to it than weight. Pay attention to weight for Kinetic energy and hunting penetration.
A much heaver arrow will load more making it change dynamically, but I highly doubt 30 - 40 grains over the entire length will matter. Changing your point weight will highly make a difference, far more than shaft weight. lowering the point weight to match the other shaft will likely make the two shafts not shoot the same. If you go by spine, stay with it.