If your going too install by height versus pressure then any thing close will work
To install by height you should buy either a decent set of inside calipers or a height mike so you can shim each spring to the same height within .005 or so
The better choice is to purchase a vice type valve spring tester and a height mike so you can shim each spring to the same pressures, + or- 5.0 Lbs. on the seats