My xm has had a xa-xc tank fitted sometime in its life , height and chassis rails arn't a problem ,but the boot hole had to be inlarged and the fuel filler is in the wrong place for the factory location and the fuel sender wont work with the factory 6volt gauge so you have to use an after market gauge.
An XR-XY tank will double the fuel capacity and allow you to use the factory fuel gauge.But you will still need to move the fuel filler pipe.
Try this link for an xr-xy fuel tank conversion from the Early Falcon Car Club of Western Australia