R100 petrol tank on a commando?

This is off a Suzuki of about 400cc. It might be close.
You might also take your tank to a bike wrecking yard if you have one and see what is close.

Many thanks Cookie, and everybody else for your input.
I saw an old honda tank on ebay. In reviewing by the pics and dimensions supplied by the seller, it seems to have the clearance needed, especially where the cradle and backbone fit together. Though it would need a little modification. if it stays cheap, I'll roll the dice on this.
Cheers,
Don

If you could solve the top tube problem then with an Interstate seat it may work, the interstate seat sarts about 3 " further back.