Shifting without letting up on the gas. The idea is to do it so quickly the engine doesn't hit the rev limiter. It is extremely hard on synchros.
FWIW, I did a quick peruse of the transmission rebuild procedures for the E350. Need to fab a bunch of special collars to press different bits and pieces on to the main and countershafts. PITA, but doable. I had to do something similar for the Getrag 6 speed, but not as extensive as what you need for the E350. Gearbox rebuilds will be a tad pricey, probably in the $300 - $500 labor range. No idea what parts costs are yet for synchros, but they are $102 for each gear in the Getrag.
FWIW, I did a quick peruse of the transmission rebuild procedures for the E350. Need to fab a bunch of special collars to press different bits and pieces on to the main and countershafts. PITA, but doable. I had to do something similar for the Getrag 6 speed, but not as extensive as what you need for the E350. Gearbox rebuilds will be a tad pricey, probably in the $300 - $500 labor range. No idea what parts costs are yet for synchros, but they are $102 for each gear in the Getrag.