Wanting to be a 400m runner and a powerlifter at the same time is extremely hard! On one hand, having too much mass will hinder 400m results while the frequent 400m work might diminish your strength gains.
Improving your 400m will depend on where your weakness is… acceleration? Top speed? Speed maintenance? Speed-endurance? It also depends on how much time you have left.
The ideal way to train for sprinting is to go from short to long. Meaning that to be fast on any distance you first must possess speed! If you don’t have any speed to maintain you’re in trouble. That’s why the method of starting the training program with longer distances and gradually decreasing to your target distance (a method often used by traditionalist track coaches) is bunk.
Since during a 400m acceleration is not that important (unless for world class performance) you should start your training by improving your top speed using flying runs (10m progressive speed build-up followed by full out sprint) of 60-80m. When your top speed has significanlty improve your build-up your speed-maintenance with 80-120m sprints. Once that this is up to par you work up to speed-endurance with 120-300m sprints, then you work on event-specific speed-endurance by running 350-450m sprints.
Each block (there 4 of them) should last around 4 weeks. So a full cycle is 16 weeks. You can cut down the duration of each block to 3 weeks (cycle duration 12-weeks) if you are pressedfor time. But I wouldn’t go lower than that.
During the weeks of a block only perform the type of running planned for that block, we want to do things progressively. However during the 4th week of each block (unloading week, in which the volume is decreased 50%) you can include a timed-tested session in which you tryto cover the longer distance possible in your target time for 400m (e.g. if you want to run a 49 sec./400m your sprint for 49 seconds, trying to cover as much distance as possible).