Remove both shift and throttle cables from the linkage. Make sure the shift linkage (on the engine) will shift F,N and R properly (without cable attached) first. Place linkage in neutral position.
If this checks out, put the shift/throttle handle in the neutral position, slip the 'hole" in the end of the throttle cable over the stud on the linkage (without moving it) and adjust barrel (wheel) on the shift cable so it matches the recess where it's supposed to go. This is a good starting point. You may need to tweak this adjustment a bit to compensate for free play in the cable.
The throttle cable should have a bit of "pre-load" on it meaning the cable should push against the linkage a
tiny bit to assure it returns to idle when throttle is at idle. Same procedure as above but make sure the throttle linkage on the motor is against the "stop". I use a bungee cord to hold the throttle linkage against the stop when I adjust throttle cables.
You didn't get the throttle cable and shift cable reversed did you?
ETA - why did you remove the cables to service the impeller? Was it an issue with access to the shift linkage bolt? I haven't done an OMC V6 in a while.