My tank is in the rear (former M724) and I looked at different ways.. none of them good. In the bed was the only decent place but at the time I was hauling a lot of gravel and other stuff... so I put that project off and ran without a spare.

In the bed you lose cargo room but it's an easy mount. Maybe plate both sides of the bed wall for support, a little tubing and another plate with a bolt or two in it to mount the wheel to.

I didn't get around to measuring the areas in front of or behind the rear fenders to see if there's room on the outside of the bed. And I didn't want to pick that fat pig up to mount it out there...

... I guess if you wanted to get ambitious you could make a swing out rack for the back (like a Rodeo or other small SUV has), mounted to the side of the bed at the back, and have it angle around the back and up against the tailgate when closed.
ooh... maybe mounted high enough that you could open it up, lower the tailgate, close it again and have the tailgate support the wheel as you mount or remove the spare.
It would have to be supported really, really well...

Somebody oughta try that. Somebody besides me.