If you mean the 4 speed manual box out of a P6B, (all P5B's are auto as standard with no manual option) then it's a very weak box to start with. If it's particularly difficult to get into second and top, and the gearlever moves backwards and forwards as you go on and off the throttle then the rear flange nut is loose, or the stud has snapped off the end of the mainshaft. This will give a wavy speedo as well. The external clutch lever also has to be set correctly. They shred teeth on the laygear very easily and as new laygears are NLA, and s/h ones very hard to come by most of the time they get changed for LT77's. Externally the selector striker levers wear, and the bushes fall off the bottom of the gearlever, all of which give loads of play and difficult selection.
Replacing the 4 speed manual with the LT77 in the P6 needs the prop to be changed to the shorter automatic one, so you'll need to shorten yours by the same amount. Prop flanges fit together without problems. If you do want to persevere with the 4 speed I do know someone who has at least one box supposed to be known good.
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