If your looking for a number stamped on the spring ain't gonna happen. The color code stripe is your only sure thing. If you can find any trace of a stripe/color you just got lucky. The Mustang Restoration Guide indicates for cars from 4/1/65;
65 HT/FB, GT, Auto trans, 1 Red Stripe. 1413 load range. C5ZZ 5310-F
65 HT/FB, GT, 3/4 speed & Auto, Manual/Power, 1 Brown Stripe. 1491 load range. C5ZZ 5310-E
The Spring Load range on all the 289 cars, in this date range, is from 1413 up to 1491. If you need to be more exact than this you will need to know all the spec's and acc.'s from the donor car. Then, there is the rear leafs to deal with. Brian