All Chev PASSENGER cars that had the powerglide trans from the factory had 3:55 gears. This torque tube to differential, as a unit will be a direct replacement for all torque tube passenger cars back to at least 1941 and probably further back than that. I'm 90 percent sure that the gears would interchange back to late 30s, while the torque tube length may be different.
And yes the splines are same, otherwise this swap would not work.
I found the hard way that the axle shafts between 49-53 were not exactly the same. The length is the same but there is a raised cicle at the end of the axle to index into the brake drum, 49s are larger and affect the brake drum fit.
Again, this is an easy swap, just read up in your shop manual and it will go just fine. Keep the stock 49 axle/brakes if you want to keep it simple, the gear change is the best upgrade you can have in your 49, all else being stock.
