I've used several of the -902 for changes and upgrades on the 7.3 over the past few years. I started using the -903 of late because of a caution about exceeding 12VDC published by Stancor on the relay. I have used the -903 for a couple installations of late because it's rated at 15VDC and the price is the same. In good shape your 12VDC system is measures about 13.2VDC on a good day with the alternator pushing about 14VDC potential when the truck is running.
I doubt it makes that much difference given how tough this relay is but figure you can order the -903 from Mouser for the same price and it gives you a bit more voltage protection. Coil resistance is like 33ohms versus 21 ohms for the -902. You just have less chance of burning out the relay coil on a warm day. Always been a lurker on the forum but thanks to all of you for the posts and information, you've made my life easier as I maintain all the trucks for our company.