Drive shaft length specs

i have wondered what the correct driveshaft length is supposed to be in my car.

68 dart ....it had a 904 and 7 1/4

i recently changed it to an 833 OD with the same rear end and i used the same driveshaft but changed to slip yoke ....it runs and drives but i think it looks to "tight" not much slip yoke visable when not in motion and when removing the driveshaft ...it takes a little jimmying to get it out. :happy1: