T 10 conversion throw out bearing question

Converting my 70 AMX from M12 auto to T 10 four speed T-Day weekend, and have questions about the throw out bearing.  I have two AMC T 10's, one with the larger diameter front bearing retainer and one with the smaller diameter front bearing retainer.  The Ram clutch kit I purchased includes a throw out bearing unit that fits the larger diameter front retainer.  However, I was planning to use the newer trans with the smaller diameter retainer.  Will the larger diameter retainer work on the newer trans (can I just switch the front retainers)?  Also, I understand that AMC used two different throw out bearing unitss, one a little longer than the other -- how do I know which one is right, will work, for my application?  I only have the longer one, and will probably try to get one of the shorter units so I have both when doing the conversion.  Thanks in advance.

Ross i am having a problem in my sc rambler. I put a jeep scatter shield 8.5 inch in it. Borg and beck 3 fork pressure plate.Sounds like it is grinding with throw out bearing with 15 miles on it. I used the smaller 69 cone but couldn't get smaller throw out bearing so we put brass bushing inside larger bearing on small cone. I would think if arm was in wrong location bearing could be rubbing on side. It looked like old big bearing has damage in front as i would expect from not fitting cone correct. I just had it apart to put rebuilt 4 speed in it. Some have said use a dyafram clutch . I had one before and thought it was weak. The car has slicks and don't mind clutch peddle pressure. I don't have money for center force. I bought a chev 10.5 kit and use there disk and pressure plate and got throw out bearing separately.   Do i use long or short bearing? i had to take the stop out inside the car to get enough throw to adjust it. Help.