Ingo-
There has been many threads regarding fuel tank relocation. A search will give you many opinions that have already been posted.
Here's my current thinking. In building up my '51 3100, I'm using a smaller saddle tank taken from a long bed with ~15 gallon capacity. Looks like there will be plenty of room for the spare under this tank. I think you cannot expect to put a deep 30 gallon tank behind the rearend and expect there to be enough room to hang the spare in addition.
Others may have contrary opinions (sometimes that happens here at the 'Bolt!!), but a smaller tank will allow room "to spare!" :p :p
