The 937 having the brass bushings, did you replace the wheelsets with steel wheels or leave the Flyer plastic wheels? I think you results are not very different from mine, however you view of them is a bit more optimistic than mine. I outright dismissed ½ car length more coasting being worth the effort.
The 937's retained the original Gilbert plastic wheels. The more force used propelling the cars resulted in a larger gap at the finish line. I would guess that the steel wheels have less rolling resistance than plastic. I'm sure you are familiar with the amount of crud that adheres to the plastic wheels.
Because I don’t use knuckle trucks I have never attempted to convert then to the brass bushings. Do you have to ream out the hole in the sideframe for the bushing? How do you ensure the bushings are all the same distance from the rail so the car rides level?
There was no need to enlarge the sideframe holes. In some cases the holes were enlarged due to wear. I believe instructions state that the bushings should be epoxied in place if the hole is too large. The difference is negligible as to ride height.