Like Mike says, any gas tank is going to see the ambient heat, so it has to be properly vented. Of course the tank between the frame rails gets direct radiant heat, but with proper venting it's not going to be any additional risk. For me it's more of an appearance issue. I like the cleaner look of bobbed frame rails in back (no, it's not my plane) as opposed to the OEM tank, especially on a highboy.
'33 & Funk 019a.jpg

Apart from appearance, the OEM gas tank is totally exposed in a rear collision, but others will argue that having it in the trunk is worse. Mine's trunk mounted, but I did put a solid bulkhead between the trunk & passenger area to at least try to delay the fire path....