It's funny because maybe 5 years ago several NOS 1968 brackets were on ebay...  they weren't very expensive.  

It might have been longer than 5 years though...  but there were quite a few that kept popping up when I was searching for a CT1 tank.

I would not butcher up a frame either . Part of the allure of the 68 DT-1 is the bolt on gas tank and small tach, without those...eh.
I have bungs that I did not use that can be welded to any decent tank and you have a usable period correct looking tank, tank problem solved.
I was lucky enough to find a great bolt on tank (think $$$$) and my 68 DT-1 came with the elusive brackett.
Yes, you will need the $500 oem yamaha tank strap/brackett, could be one on ebay here and there.
Welcome to the treasure hunt called restoring a 68, you will need a treasure chest to make it happen, ask me how I know

I fabricated fuel tank mounting brackets for my 1968 DT1. I did not want to weld, drill or modify frame to use 1969 thru 1971 tanks.
The brackets are also spaced out from the frame, as the 1968 is narrower in this area.
This allows me to easily remove if and when a 1968 tank is used.

 

Nice work!

Great job and nice fabrication work too!