mav the DT400B is a 1975 model, regardless of the date of manufacture. It was not a 1974 model, that was the DT360A. Now, as far as the rules go, there is allowance for 'follow on' models but this has only been ratified for some bikes which are specified in the MoMS. The DT250B is a follow on for the simple reason there was a DT250A and there are few differences between the two.
There would certainly be opinions one way or the other, but at the end of the day the bottom line is that the DT400B is a 1975 model that is not designated as a follow on. It is therefore ineligible to race in the Pre 75 class.