Driveshaft help

First verify if the 727 trans is the same length as the 904. If it isnt then you would be best off waiting until the trans and axle you intend to use are in the car and the car is on the ground. The measurement according to the mopar chassis book is the distance from the outputshaft of the trans to the Center line of the differential u joint car on the ground, then subtract 3.5 inches, for the u joint center to u joint center dimension. You will need a different trans yoke as well - hopefully this is useful.