[Lynn Eades]

Richard F. has it right. The brake shop will take the minimum required out, and thickness the oversize linings by radius grinding them to fit the drum.
Check your wheel bearings carefully, fit new inner hub seals, and make sure you remove the tin shields from the axle (6 castle nuts each side) clean inside them, and make sure you clean out the "tell tale" holes, through the backing plates. This shows you if a hub seal has started leaking, so that you can catch it before it gets to the new linings.
Note. the rust pitting has happened to your drums where the linings don't touch them, when your carrier has sat for 25 years at a time. This rough bit tears your linings to bits.
While you are waiting for the drums and linings to get done, I would strip, clean, and re assemble your bisectors (making very sure you assemble them properly. If you get the wedges in the wrong way around, very little travel will be the result. Use a good grease, and be very careful with the alloy housings
You might advise the brake people that this job would maybe be better done with low speed (woven?) linings.