8 1/4 axle specs??

Listen guys....I am new to the A bodies and have had a ton of questions. I get on with you guys and find a hell of alot. I have a pretty stout 340 in my 73 Dart and was thinking about trashing the 8 1/4 and going with the 8 3/4. So I watched the video and saw you hammer the Cuda. I have a 4 sp car......you think I should stay with it??? If i was to go with the 8 3/4, what dimensions should the 8 3/4 be from flange to flange? I want to make sure that I am getting the right one!!