Well Bill, still working it out but think it is doable. Here's the situation, my Manx tank is longer than the stock tank by about 3 inches if that. Both fit fine on the front end. The back is the trick. Im leaning toward having my upholstery craftsman attached the seat with snaps to the fibreglass, when I have it done I will have the craftsman make an add on piece that matches. I haven't thought out just year how to mount it easily to the frame or seat, thinking just now what comes to mind is "Velcro" have a flap sewn underneath onto the add on piece which could easily be attached to the mating part underneath the front of the seat? It wouldn't need to support much, maybe ones balls likely nothing at all. Gas line wise, the Manx tank has two petcocks so will need 3 double sealing quick disconnects, the stock tank only needs one then 1 matching opposite disconnect on the carb line. Click, click, click, swap, click new tank done. Oh as far as the mount in the back go's the stock bike uses the stock rubber, which I remove and add a one screw bracket to hold the back end of the Manx.
Didn't do anything today except store the stock tank away. Worked from 9-3, then had to run the dogs then take the lady of the house shoe shopping and whole foods shopping, but she did buy dinner and a new pair to tennis to where with the ugly shorts she bought me for going out this summer, Cant wait for that. LOL
Hope your thawing out up there its been in the 70's and 80's during the day the last week or so. Stay well!
gm