To mount the tank "correctly" or easily, you will need to weld a bracket - an upside-down 'U' to the rear of the frame.
That will need two studs coming upward, to receive the bracket on the /5 tank.
Then you use a couple of thumbscrews to tighten it down, Type 247-style.
In this image of a /5 tank, you want something like #s 8 & 9.

For #9 you could use the rubber bumpers for the R65 battery box mounts.
If you can weld, that would really help.
Perhaps one of the other posters that has a 247 will take a picture of it for you. I am not having any luck on Google.