Weeds, first of all, what will be your rear seat arrangement? What size wheel and tire will you be installing? From what you state the new gas tank location is where the original spare tire went. Is that correct? If so, you need to find another spot and I don't think there is any other place underneath that will work.
Your original spare tire carrier for the 49, was the drop down type, different from the later carriers. Your's , was attached with a pivot at the rear and a j-hook that was accessible from just inside the rear doors. The later units were diagonal across the rear from left to right.
If you choose to use a space saver spare, you can probably mount it on the inside left rear. I would put it here instead of the right side, because the right side is where you will most likely go in and out of the most. Another idea would be to attach it to the rear of the rear seat.
Another idea is to join AAA and get roadside service, then you don't need a spare at all!
If you could provide more info, maybe others could give you better input.
