That is the bearing support pipe plug. If you just loosened it or removed it without driving it loose you are probably ok, if not it could of slid out of the case towards the ujoint then it could just spin with the mainshaft causing the mainshaft to whobble in the case. When I disassembled my 3 sp that bearing support slid out really easy,not a press fit. It has been awhile but I seem to remember it had a 3/8" indentation in the bearing support for the retaining plug to center in.Since there is no seal in there, you should be able to view it if you removed the torque tube and ujoint and iinput yoke. Then you could recheck your ujoint to see if it is ok too.

Last edited by sstock; 04/30/2018 4:44 PM.
1953 Chevrolet 3100
261 cu inch, sm420, 3.55 rear, torque tube still,omaha orange, still 6 volt, RPO green glass, side carrier spare, all done
In the DITY Gallery
Video of the 261 running

1964 GMC 1000
305 Big Block V6, sm420, the next cab off restoration