TR, in your situation, all is ok until you put the shift rail in?
You can "locate" the first/reverse slider gear by hand at a point where it doesn't rub on either the cluster's first gear or the reverse idler gear, but after you put the shift fork, the shift rail, and the poppet ball in, then it rubs?
Alan W. Johnson
MB 201453, Jan '43
M-100, Sep '51
MB-TD, Mar '12