I am working on putting a 261 in my '50 right now. The main difficulty is shortening the water pump to fit behind the radiator. I got a junkyard pulley from an F-truck, around 68 or so, with part number C8AE-8A528-B. It has the same bolt pattern, belt, and shaft size as the stock 235/261 chevy pulley with 1-1/4" less dish. You can press the hub on the water pump to the right position and cut off the excess shaft. If you don't have a press you can rig one up with a bottle jack, a piece of pipe, and the frame of your truck.
If you want to keep the truck 6 volt or use the original 216 starter then you need to do something with the flywheel. The 261 will have a differant tooth count and diameter on the ring gear. I had a machine shop machine my 261 flywheel to accept the 216 ring gear and it turned out pretty good and cost me $50.
You can use whatever bell housing fits, since the 261 has the 216 style bolt pattern. The front motor mounts may need work depending on the year of engine you have. You can drill the front plate of the motor to match your old one, or get a '60-62 front plate because they went back to the 216- style plate. I was able to drop mine right in, since I have a '62 block.
Be sure to install the oil filter, since most 261's are full flow, but I don't know about canadian blocks.
You may also need to adjust your throttle linkage a bit. That's about it, it's pretty easy as far as engine swaps go.
