As seen on this post from ADVrider:    advrider.com/f/threads/dt400-to-supermot...-legal.866081/page-2 

I'm wondering if this would work on a '76 model... And if it was just bolt-on, as the guy says. 

I have not done this mod or heard of it. 

It does not surprise me that he said the triple clamp was a direct bolt-on.  Yamaha used the same steering head tube dimensions, races, and loose ball bearings on many models. 

With the TW not having leading axle forks, you shouldn't have any trouble with triples hitting tank.  If what you want is a wide 18" front wheel it might be a worth a try.

Thanks for the info Mark. That’s pretty much what I was hoping to hear when I came here for a second opinion.  

And yes, the plan would be to lace a 2.50x19 Excel rim to the TW hub, then throw a wider flat track tire on it.. (at least just a bit wider than the stock DT setup would allow..)  

I will give it a go and reply here with my findings. 

TW forks in the Link pic i see has leading axle so he must've used the TW triples or the offset will be all up the creek. So must've built up steering stops or it'll be hitting the tank. Looks interesting--ie--good though.

Turns out that 2001+ TW front ends are a bit more scarce than I expected. In the meantime I picked up a dirt cheap 87 TW triple just to confirm that this is doable. I threw it on the bike and happy to report that it was straightforward - basically bolt on. Forks/triples have plenty of clearance to the gas tank. The only modification I had to do was shave down the TW's steering stops just a smidge - they are taller than the DT's and thus they make contact on the frame. And the TW stops are sloped instead of a flat, which doesn't match the frame perfectly but I don't think that is too big of an issue. 

I also got my hands on a 01+ front hub which will be sent out for wheel building while I wait patiently for forks to appear on ebay 
I'll post pics when it's all buttoned up.