Seems like too many comments and not enough answers!
You need to do backstroke along with freestyle, because:
- Backstroke covers certain muscles a lot better than freestyle, and
- Gym can't perfectly replicate backstroke swim time.
- If you're going to swim as much as you do, you may as well get comfortable with a second stroke.
Doing different swimming strokes prevents RSI. Don't think of backstroke as wasted time - you're still improving your cardiovascular capacity, and many of the muscles cross over. E.g. backstroke will improve your shoulder flexibility and your back muscles, which will probably help with the arms out of water cycle of freestyle. So, don't be afraid to actually sacrifice some freestyle time for backstroke. By the way, I'm a devoted freestyler, but I know the importance of this "cross-training".
Also, you need to be ready for that glory moment when Coach tells the team that the star backstroker is injured and can't swim the medley relay, and you put your hand up and say "Put me on, I got this Coach!"!!!
Getting back on track... some backstroke-esque gym routines are still great to do:
- Rows, where you bring your arms out back and wide thereby extending pectoral muscles - this is a nice opposite to the heavy pectoral work that freestyle does, should help build a good range of motion.
- Rotator cuffs in the direction opposite to freestyle. Free weights would probably be best, basically just keeping your upper arm steady and rotating as your forearm lifts up.
Honestly I think that's the bulk of it, it's really about the reversed shoulder motion and the back-focused rather than chest-focused work. If your freestyle stroke is good, both your biceps and triceps would be getting worked hard. Kick is fairly equal on both hamstring and quads.
Align the reps and sets to how you swim freestyle - if you mainly sprint, build power, if you mainly do long distance, build endurance.