It would help if we had an idea of what you plan in doing. If the thimbles are going on a shackle, are they large enough to slide over the bow of the shackle? I thought they were going to have fixed eyes on both ends, but after rereading your 1st post, it sounds like their going on the shackle that is attached to the load line hook. If that's the case, you can tie hitches on to the load. Then the thimbles would have to be large enough to slide over the 'bow' of the shackle. The pin should ride on the hook and the spider legs on the bow. (see attachment)
All of my spider legs have 15" or longer spliced eyes with no thimbles. Most of them have a 'sleeve' in the eye to help protect against wear. That may be a better route to take. It's a whole lot less expensive too.