The picture you saw that goes through the uprights is most likely a cadet. They used a pretty standard Reghetti Ridolfi tank from what I remember, you probably just need a smaller tank than what is on the kart. The person that would know for sure is Greg at Leading Edge Motorsports, they were a pretty prominent TrackMagic dealer.
Thanks for the replies everybody. I must not have a Diablo then, because the KG 8.5 is the tank I most recently tried and rubs just as badly as the one that came with it: https://new.ekartingnews.com/forums/topic/2-shifter-kart-fuel-tanks-7-5l/#post-33906
I have a 1995 Trackmagic and had the same problem with my tank. I then bought a traditional 8 liter tank with the bolt hole built into the tank, made to screw on a bolt through the uprights. I welded a small plate between the uprights and drilled a hole through the center for the bolt to go through. Fits like a charm now.
100% its a 2000. The First year of the Diablo. 99 would be a Demon. Then you move on to the Diablo II, III, IV and then the FTR from 2005-07. If you look at ads on Craigslist very few know what year they actually own.