The DS for the FW is longer than the B body (Caprice/Impala). This means that it runs into a problem with critical speed at a lower speed than the shorter-wheelbase B body. Since the shaft is so long and skinny, the critical speed will easily be reached by a stock car with the speed limiter programmed out. I would suggest an aluminum shaft from Denny's, and get it 3.5 inches (4 if you can get it to fit - and some have on B bodies). The larger diameter with effectively increase the critical speed of the shaft, allowing you to sustain high speed with no concerns. Also, Denny balances shafts at 8000 rpm (double what most others do) so it will be smooth as silk at all speeds. You can also go with metal matrix or carbon fiber, and specify larger U joints if you think you need them. Call Denny's - they will help with all the selections.