I came into a set of Wheels off of a 78 IT400. Does anyone know if these will swap over to a 78 DT400. It sure would be nice to have a extra set of wheels for my DT. 
Thanks

A good start would be to see if any of the component parts are shared - this can be can by looking at an individual part on the CMSNL site and scrolling down to the list of which bikes it was fitted to (maybe Partzilla as well, but their records aren't as comprehensive).

Ok I went to CMSNL and ....

After further review, the backing plates are different front and rear. And the DT uses a rear rubber cush hub and the IT does not. So its looking like for a swap I would have to swap over all the brake components too. So, not impossible but not a easy swap in regards to just wanting to use a different tire for road or dirt use. Ill just look for another set of DT wheels on fleabay.

I think you're correct that you might need "complete" wheels with backing plates and speedo drive, especially on the front.

Just wanted to mention that the IT models often have magnesium backing plates with a different part number than the aluminum version but can otherwise be physically identical.  I've even seen a completely new part number for a part when the only difference is the shade of the backing plate paint.  (Yes, there is a "color code" on parts but sometimes Yamaha changes the part prefix)

Which all a long way of saying that sometimes you need to physically compare the parts, Yamaha "recycles" many parts over decades and models.  You can even find parts on brand new bikes that will fit the 1970's models!

And again, I think you're making the right call here.