Greetings. 
   I want to adapt a 1969 thru 1971 fuel tank to my 1968 DT1. Does anyone have a way to do this without modifying the frame. I want to keep the frame stock so I can add the correct tank later. 
THANKS IN ADVANCE. 

I thought they were all the same mounting. Frame changed a little in "72 & tank too with a flip cap, but even then they probably interchange. Tell us more or correct me please.

The 1968 tank and frame are unique in that it does not use brackets welded on the frame for the gas tank rubber mounts to slide into. If you look at the parts breakdown you will see the 1968 tank bolts on with a bracket going horizontal under the bottom of the tank. Later tanks do not fit on the 1968 frame. I'm currently working on an adapter bracket that will allow me to use a later tank on the 1968 frame.
Thanks for your responce

It is a fairly simple effort to add self-fabricated cups to the frame of the 68 models, so they accept the 69-70 gas tanks. It does require that you drill two holes into the frame to bolt on the cups in the correct place. I did not do this but my 1968 DT1-250 had this done before I took ownership. Wish I had a 68 tank on hand but if I did, I would still just put it on it on the shelf and leave on my 1969 fuel tank as it is much simpler to remove and reinstall. I would post a picture, but it seems we are still under a space restriction. 

DEET once posted using automotive cup-type freeze plugs of the right size and notching a section out of the edge to make puck holders you can bolt or weld to the frame.

I learn every day haha. December "68 we got new had a gold tank & normal cup fittings. Am aware of the under bracket talked of but thought it still had cups & just the under as an added support. Good to know thanks.

I would think to replicate a '68 tank you could use a couple of rivet nuts in the bottom of a later tank and braze them in . You could then avoid the frame cups. I think later tanks have the protrusions like a 68 tank, at least my '71 RT-1B tank has them.

I bought some bungs from a bungs.something? (website) , in the correct thread size to be used on a tank as I had intended to modify a tank for my 68 DT-1. The threaded bungs are available but you would need a correct tank as a pattern as to where to weld them probably. 

I feel it’s a bad idea to cannibalize or modify any frame. With that being said, I would have a later motorcycle frame that had the pucks. I would not destroy a motorcycle to make it to my liking.

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Currently working on a '68. The reproduction parts manual I have has corrections in them that would seem to indicate that Yamaha changed mid-stream that year several times .

One change was to how the tank was mounted. Initially a shortish "U" shaped bracket was bolted to an underbrace. This was then bolted to the underside of the tank. A guess would be that that system was unsatisfactory, and the later system with the rubber dampers (the one that we all know and love!) was done mid-year.

If one had a '68 frame but no tank, one could obtain a later '69 and up usable tank. The 'cups' talked about could be mounted up to the frame appropriately, and the later tank slipped in place. The 'cups' are not seen. If the "correct" tank was later found and one wished to use that, you would just swap the tank and mount it as originally intended, leaving the cups you had installed alone, or removing. They would still remain unseen (only your hairdresser would know!)

That "U" shaped bracket is currently stupid-expensive should you need one . OEM only unless Yamabob had some made.