Take a close look inside the rear of the crank on your old motor. You will find that there is a bushing inside. It is not thick like a manual trans pilot bushing, but just a thin guage spacer bushing. The th400 torque converter has a smaller nose on it than the 999 torqueflite. Either remove the old one(good luck!!) or purchase a new one. I would suggest the dealership. Or a good machine shop could make you one. I ran into this problem when I installed a 360 from a cj with the th400 into a cj with a 999. I couldn't get them mated together because that stupid bushing was there. Took about an hour to figure it out. Hope this helps, Hamy.
One Way Up Racing