Denis,
The primary shaft is stepped down to locate a single row of rollers - the outer track is the same width and trapped between hardened thrust washers, so the length of the rollers will determine the endfloat on the primary shaft. Personally, I wouldn't worry about 25 thou of endfloat providing top gear selects correctly. The endfloat is not likely to affect the operation of the oilseal as the movement will be axial - the bronze bearing between the primary shaft and the mainshaft controls the amount the primary shaft can move out of alignment. If you decide you want to reduce the endfloat, this can only be done by fitting longer rollers - but beware, 10 thou longer rollers will reduce the endfloat by double that amount. You should not attempt to achieve zero endfloat - that will probably result in the front bearing running tight and seizing up. In any case, it is not possible to adjust the endfloat by shimming the retaining plate, but the increased endfloat could be caused if a previous 'repair' has resulted in the retaining plate being dished. If this is the case, Postwar Spares should be able to help with a replacement.
Regards,
Eddie.