I am in the processing of rebuilding an 904 also..this is what i use for the rear servo...

http://www.makcotransmissionparts.com/S12912.html

it has wider lips to keep it from flipping in the bore...have used 2 or 3 of these already.

Tony

70 AARCuda Vitamin C
71 Dart Swinger 360 10.318 @ 128.22(10-04-14 Bakersfield)
71 Demon 360 10.666 @122.41 (01-29-17 @ Las Vegas)
71 Duster 408 (10.29 @ 127.86 3/16/19 Las Vegas)