I would consider a custom roll bar that the spare can mount to. I envision the tire mounted straight up, just behind the cab, raised up enough so that plywood, etc. can slide underneath. Maybe raise it up about 6". You will still lose bed space but the tire will be secured far forward out of the way. I have seen trucks with a similar set up years ago. There is an company in the Gainesville area of FL that makes real nice custom roll bars. I have the web address. I'll find it for you.