ORIGINAL: touch and go
I think it probably would have an adverse affect. Whenever you had (1) the plane in a nose high position for a significant period of time and (2) the fuel level in the tank somewhat less than full, then the engine would probably quit due to fuel starvation since the fuel would be at the bottom and rear portion of the tank, due to gravity. IMHO putting the clunk forward should not be done.
Well honestly.. that depends on how one flies his plane...
Unless you make REALLY LONG power dives, putting your tank in backwards will have very little or no ill effects.
And, as long as you make sure the clunk follows the fuel, no matter what you do, then there's no problem...