What both JA and your guy are saying is that if you want to change the gear , you have to change the set , as they have to mate up for a proper match [ all mesh gear assem. are changed as sets- if you change one only, the other one will wear the new one out in short order, as the old/new contact surfaces will not mate correctly ]
And the new improved gears set requires a new thrust peice b/c the new one is a different dimension than the original [ allowing for the larger bushing surface/shoulder on the new gear]
.. but , if there is not a lot of wear on the gear assem, there is nothing wrong with just replacing the thrust piece with an original replacement [ or shimming]
The modified setup will not wear as the original does, but it takes a long time to wear one out and the original can be re-shimmed much cheaper than the mod. cost without the fear of of early breakage or oil pump drive problems .. it is just that one should do a routine end play observation when the miles get high on the replacements , even if it is the new mod....
If the gear/shaft look Ok, go with the thrust piece, but if the gear is bad , go with the complete gear set and matching thrust piece.
For those who prefer the shim tecnique over the replacement of the thrust piece, it is preferable to press the bushing out of the thrust casing and insert the shim behind the bush , as it is then sandwiched inside the unit, allowing for any type shim stock , as the original brass bush material is still the bearing surface.[which you resurface] With a shim on top of the thrust, not only is the shim wear qualities in question, but you can then get wear on both sides of the shim as it floats between the gear and the bushing surfaces. As the wear becomes larger , the gears take the extra wear as they start to lift with each drive input due to the rotational cut of the gears. So, smaller end float with best material is better all around.