Somebody correct me if I am wrong here on any of this, but I believe the 1954, 239 Y-block intake and heads are smaller than the later years and are NOT interchangeable.
Beyond that, the 292 truck intake gaskets are different from the car engine intake gaskets but they ARE interchangeable with the 272, 292 and 312 engines, I think. The only difference in the truck intake gasket is that is has a smaller opening at the heat riser passage, but mating between intake and heads is identical.
There is no difference between the left and right intake gaskets. They are identical. The exhaust manifold gaskets are also identical from side to side, but they have aluminum-facing on one face of each gasket. You want to install the aluminum-faced side outward on each.
2V = (2) venturi intake. 4V = (4) venturi intake. I doubt there is an aftermarket aluminum 2V intake available. They probably have a 4V and possibly a dual 4V for hot-rodders. Pick the poison you want to use. It does not make any difference which carb you plan to use. The mating between intake and heads is the same as I described above.
Sorry, I don't know that much about the Mercury 256, but it looks to me like a 272 with a little smaller bore maybe. Ford used the little 239 in trucks from '54 thru '55, while the 239 was discontinued in cars at the end of the '54 model year. I think they started using the 292 with a steel crank in trucks from '56 thru '64. 1964 was the last year for the y-block engine.