I think they are not. As far as I know, the yoke needs changed to do that swap.

All driveshaft lengths are posted in the AMA specs. Click on the link at the bottom to find the page for your year.

(it doesn't say how to measure there either, but you could use your current shaft as a guide. i think it's from center to center of the ujoint yokes... center of where the ujoint caps go...)