That's where the measuring comes in. Get dimensions of your frame and mounting options. How wide does the tunnel need to be? How long is the stock tank nose to tail? Then ask someone that has one of the tanks you are considering to take some measurements and see how close the 2 are.
Things to think about: where is the petcock on the non stock tank? It may be in a bad place for you in the new arrangement.
Clearance with your fork legs could be an issue, and the tunnel is a likely sticking point.
Minor adjustments can be made with a hammer and dolly if needed.
Cutting and welding of tanks is not generally novice work, and pro built tanks are awesome but expensive!