I have looked at many sites trying to find a smaller diameter set and had no luck They all seem to be the same size. If I had that frame and tank set up, i would look at two headlight housings and mount them inside those like instruments, whoever it was that mentioned that idea is a genius. I may go that route myself
Another option since you have the ability to fabricate a tank, how about making the sides follow the lower frame tubes and then come up an inch or two over the top tube. Then the gauges could be mounted in a row from front to back
Not trying to tell you what to do here, i have been trying to figure out how to mount a set myself for at least a week. My top tube is straight but Im doing a chopper and just gotta have a tank on it
Good luck with your build however you do it