The dashboard CAN controller creates the CAN bitrate from the 5MHz crystal. This is done by the internal hardware and you do not need to worry about it. Only the 50kbits/s you measured are important.
Your MCP2515 needs to use 50kbits/s. To configure the MCP2515 correctly you will need to tell the library the frequency of the crystal. If you tell the library the wrong frequency the bitrate will be wrong. e.g., 16MHz vs 8MHz will cause 100kbits/s instead of the 50kbits/s.
The serial baudrate is independent from the CAN bitrate.
Sounds promising. Can you receive CAN messages now?