One source of slop and mushiness in the shift linkage is the selector finger bushing. Side loading and grit will eventually wear out these bushings. This bushing fits Webster-manufactured Mk-series bearing carriers. Two bushings are required. Sold individually. These bushings are designed to be a very tight fit. You may need to ream or hone the ID slightly to allow your selector finger to move freely.
We also carry the Selector Finger Bushing for Hewland Mk9 Bearing Carrier, Part No. 1410-A19H.