I've got a yellow-tank, DT400 which is 501-001684.
From what I can tell, in standard form, they're fat and slow for a 400. I seem to recall the words "bloated" and "leaden" being used...
As a VMX race bike, they're not legal for pre-75, and too old to be any good as a pre-78 bike.
Everyone seems to say that they'd make a reasonable dirt-track bike, but I haven't ridden it yet, nor have I ridden dirt-track...
I'm keeping mine and putting it on club rego. It'll be fitted with trials univerals and won't be expected to do anything more difficult than fire-road runs (although if the next Vinduro is more friendly to fat old trail bikes, it might get used for that).