Anything will be an improvement!
My DT250A took a stock standard YZ400D carb without any complaints - made it go a hell of a lot harder and the YZ400 jetting was close-enough that I was happy. The carb outlet had to be jammed into the reed boot, but it wasn't a drama.
I imagine that any MX/YZ250/400 carb of that era will be a big help and will have usable jetting (rather than having to start from scratch).
The standard airbox will almost certainly have to go - 'all' of the decent carbs have a much larger intake than the standard DT carb.
Edit: Some DT400s came with itty bitty little carbs, and matching itty bitty little reed boots. If you're unlucky enough to have one of them, then you'd want to chase up the larger version.
I almost certainly have a spare MX250A airbox, which is heaps better than either a pod filter or a DT airbox. They take an old-style 34mm carb too. No airbox-to-carb rubber though.
