Look at the components of the 400 as this: acceleration, max speed, metabolic conditioning, and strength to maintain form after you’ve passed the lactate threshold (second 200).
Acceleration isn’t as crucial in the 400. No one in the 400 every worries too much about their block starts. This is because sufficient acceleration will come naturally just from the top speed training.
Metabolic Conditioning is available to everyone through their interval training. This is really just a question of how hard you push yourself in interval work. No one can improve on this area drastically over anyone else. We’re all fairly similar chemically. After a certain amount of time (~30 secs) we’re all going to be completely saturated in lactic acid.
The real trick in the 400 is in these two remaining categories. As I mentioned, if everyone is hitting lactate thresholds at about the same time in seconds, then the person who can get further in that same amount of time with the same amount of perceived exertion is going to do better. This means improving your top speed. If you ran all out against Jeremy Wariner, you’d both start “feeling it” at about the same time, the only difference is when you start feeling it, you’ve just passed the 230M mark, whereas he’s all the way over at 280M.
Then of course, once everyone hits that threshold, those who have the core and overall strength to maintain proper form through those last 150m will be able to carry their speed much better to the finish line.
So during the off season, hit the weights. Get your max strength and explosiveness up, because as you mentioned, this will increase your explosiveness on each step, thereby increasing your stride length and your top speed. Looking at your strength numbers, you’ve got a lot of room for easy improvement in your strength. Hit the posterior chain, and your reactive strength (quads, calves, and plyos). As far as max strength vs. hyper trophy, you’ve got to cycle those. You’ve got to focus on max strength for a cycle, and then focus on volume for a cycle.
Then also get your core strength up to maintain form in the last 150m. If you’re upper body is a big wet noodle in the last 150, there’s no way your legs will be able to keep their running form with a big floppy weight on top of them.
Continue running workouts in the off season, but I’d try to focus more on the form and speed, just so your form adapts as your putting on muscle weight and strength. Of course also maintain some level of sprint conditioning, but I wouldn’t worry about really hitting the interval work hard until closer to the season. The real improvements right now can be made in top speed and strength.