What tires are you running to know the difference in height between front and rear?
Why use the space saver wheel? I'd just get a modern skinny space saver spare of the correct overall height and 5x4.5 bolt pattern. It doesn't even need to be 15" dia. as long as the total height matches.
And, I drive my Pantera more than average (2,500 mile trips) and have never seen the need to carry a spare. Yes, I have had a flat and still don't carry a spare!