Gerry,
I had this same problem with the 3-speed in my 40 1/2 ton. Note that all the 3-speed car and truck transmissions from 1940 through 1952 used the same internal parts. In looking through the 1949 to 1953 shop manual, I noticed a section in the transmission chapter titled "High Gear Disengagement" See this:
http://chevy.oldcarmanualproject.com/shop/1949_53/07trans/7_010.HTM. Evidently, it was not unheard of for these transmissions to pop out of high gear. And, in fact, my 40 was doing exactly what your 46 is doing, and would eventually pop out of gear, both when drifting down grade, or when pulling hard on a long upgrade. This presents a real danger of over-revving the engine when it pops out of gear going up hill.
The manual suggests three possible causes: (1) Insufficient spring tension on the shifter detent ball spring. (2) Excessive looseness in the gearshift control lever. (3) Misalignment between transmission and engine.
In my case, the force attempting to disengage high gear was so great that it would even pop out if I was trying to hold the shift lever in gear. From that, I decided that the problem was not excessive looseness of the shift lever. The tension of the shifter detent spring seemed normal, so I guessed that the problem was misalignment of the transmission to the engine.
Inspection of the gears showed noticeable wear on the high gear teeth of the main shaft (also called input shaft or clutch gear) and on the mating internal teeth of the 2nd/3rd synchronizer clutch gear. I was surprised to see that these teeth only engage about 3/8", but that is apparaently the design. The excessive wear on these teeth pressure faces was cetainly related to the popping out of high gear problem, but the question was, is the wear the cause of the problem or merely a symptom of the problem? In either case, I needed to replace the two parts. I was fortunate enough to locate a NOS input shaft at a great price through a classified add in the G&D, and a good used synchronizer gear through another G&D classified add.
Rather than simply reassemble and call it good, risking ruining the new parts, I thought I should check the alignment between the transmission and the engine. If you go to the site listed above, it gives a procedure for checking the alignment by using a dial indicator to check the concentricity of the output shaft bearing bore with the crankshaft. If not concentric, shims are added between the transmission mounting ears and the bellhousing until the dial indicator shows concentricity. It's not the easiest thing to do and requires considerable patience. Note that in making the special tool to check the alignment, you do not have to grind the splines off the input shaft if you remove the clutch disc and pressure plate. It's also not necessary to weld a rod into the main shaft--a 3/8" dia rod fits in the hole, and if you upset the surface of the rod before installing it, it will be tight enough to work satisfactorily.
I did find some misalignment in mine and added .010" under one foot and .005" under two other feet. It didn't seem like enough misalignment to cause the problem, but I didn't want to take any chances.
A simpler and more basic alignment check, which should be described in the shop manual for your 46 is to check that the transmission mounting surface of the bellhousing is exactly perpendicular to the crankshaft axis and that the bore in the bellhousing for the transmission mainshaft bearing retainer is concentric with the crankshaft. These two checks are done by mounting a dial indicator to the flywheel (usually with a magnetic base) then rotating the crankshaft while the dial indicator is running first on the transmission mounting face of the bellhousing, then on the inside diameter of the mainshaft bearing retainer bore in the bellhousing. These two checks should fall within the limits specified. The fix for parallel misalignment might be to check for dirt or debris between the rear of the engine block and the bellhousing. I guess the fix for radial misalignment is to get a new bellhousing.
Since correcting the alignment and reassembling the transmission with the new parts, I've put about 4,000 miles on the truck here in the mountain west, up long hills and down, and have not had any further problems with the transmission popping out of gear. The shift lever stays nice and calm, like it's supposed to. I'm not 100% convinced that misalignment was the problem, but the problem appears to be cured.
If you had your transmission rebuilt, you should aske the rebuilder if he replaced the input shaft and the synchronizer clutch gear, and if he checked the transmission alignment.
Mark
