Dan,
I own a 68 Sport, same set up as yours . . and considered the same change . .
Let me share what was shared with me by previous owners and racers of these boats . . . factor in that this info is based on a boat with twin small block V8s which is how I will leave mine.
All felt that the original position allowed the boat to fly horizontal when airborne, and was best blanaced as built. Some also had added more tanks for long distance races, tanks further forward and aft tanks under the cockpit. They all strived to empty these tanks prior to emptying the tanks in the stock location.
Many boats have been changed to single engine configurations, possibly lightening the stern weight ??? and in this application I could possibly see a change in tank location.
My guess is that you'd like more space below . . . ?
leaving the tanks as built will also allow me to alter the cockpit floor, where I'll lower the floor ~ 8" behind the steering and passenger position, allowing me to stand much more comfortably while driving.
See this thread for some info on lowering the cockpit floor:
http://www.donzi.net/forums/showthread.php?t=54221