2-cable 904 trans swap

After an extended park in the driveway, the Valiant is getting a new 904 next week. With the new style slip yokes, how do i measure how much to have the driveshaft cut down? Is it a simple measurement from the rear-end u-joint to the new slip yoke u-joint?
I will have my pushbutton selector, cables, and trans up for sale very soon. Also have a couple sets of factory manifolds for LA-V8 and some odds/ends things.