It is always best to use standard wiring
w/ standard receptacles / plugs.
The modification under-discussion would create a non-standard wiring. The change does not appear to create a safety
problem. An adapter would be a better solution.
The wiring diagram for the two plugs are shown.
For both plugs, G is ground, the green wire and W is neutral, the white wire.
The other pin(s) are the line connections, X and Y are L1 and L2 on the NEMA L14-50, the opposite phases of the 125VAC line voltage, those would be black and red wires. The unmarked pin on the NEMA L6-50 is the single
125VAC line voltage, the black wire.
Yes, the adapter would take X or Y and feed that to the line input on the power cord.
NEMA L14-50 - 125V/250V / 50A
NEMA L6-50 - 125V / 50A