Transmission Output Shaft Question

Hi all,
I have a A999 lock up transmission I just picked up and I did a search to find out what i that would need to change and be aware about when changing from my original 904 in my 72 Scamp . There's not a whole lot different but i know i would need a 1968+ crank for the converter. I think i read something about the output splines for the driveshaft slip yoke are different. Are the 904 and 999 output shafts differently splined and if so where can i find the proper slip yoke?
Are the slip yokes the same length so i can reuse the driveshaft?
Thanks