I had an issue where truck was jumping out of gear. Same spec truck and box. Only realistic solution for that was recon box. About £3500 for box and fitting from my local dealer. Think they done it overnight too, so no downtime.
Rang a few of these ‘gearbox experts’ who adverise in trade press, and it was all ‘drop it off and we will take a look’. Once you calculate any potential savings against the loss of work, and inconvenience, dealership seemed the best option for me. Not had a problem since, and the only problem I have had with it in 5 yrs of running it.