Interesting serial #. At the end of gas trap production and the changeover to gas port barrels and gas cylinders. In Billy Pyle's book "The Gas Trap Garand", there is a data sheet on rifle # 50313, made in 7-40. That's very close to yours.
Your rifle started out life as a late type 3 gas trap rifle. A barrel that was made in July of 1940 is going to be a gas trap barrel, and will be virtually impossible to find. And if you do find one, the only gas cylinder that will work on it is a gas trap cylinder,(Type 3), which is damn near LITERALLY impossible to find. Then there is the gas plug, the screw and the front sight.
I believe you said you wanted a shooter. You will have to get a gas port barrel and use the newer parts to build a gun. Far cheaper, easier and you will have a better gun. The Army recalled and rebuilt all but a handful of the 50,000 or so gas trap rifles made up to then. They got almost all of them, less than 50 gas trap rifles are known to exist today, about half of them in private collections.
One sold at RIA Company last year for $38,000, and it was WASTED. it looked like it had been dragged behind a truck on a gravel road!
Good luck on your project.