We used grease and made a dummy shaft A few thou smaller than hole in front of the case.This holds needle bearings and washers in place.Then you can drive the shaft in from the back and pushes the dummy shaft out.
I took a saginaw shaft and cut it so the ends are just inside your cluster then when you install it in the case it slips right past the two thrust washers then slide your muncie shaft in and the dummy shaft out.Tore my share of muncies up,i learened to shim the input gear back about .020 or so usually to get it in good mesh with the counter gear,the shims i used were 10 bolt chevy pinion shims worked great.Also take the slop from the counter gear out,by shimming under the front thrust washer which will move counter gear back.
Get a wooden dowel same approx diameter of shaft,cut it the same length as the cluster, install the needle bearing ,insert cluster in case ,slide in shaft and it will push the dummy shaft out
Easiest with a dummy haft just shorter than the cluster. can't drop a needle bearing that way. I've always used a shaft with grease. jerry