Conversion driveshaft /6-904-8 3/4
Look in the FSM and the parts book. The CL to CL dimensions are there.
But measure twice cut once.
Best is to assemble with /6 904 and after 8 3/4 is in, get it on all 4 tires with typical weight ( IE seats, gas in the tank etc) and measure. If the measurements are correct for what is posted here and in the FSM / parts book, you are good to go.
I assume you will be using a 727 with the 440 and that tail shaft MIGHT be different with respect to drive shaft length.