If the LCD uses SPI too then both devices will share the MOSI, MISO and SCK pins but then have a SS pin each. Hardware SS is usually pin 10, so that is probably your pin 54 equivalent on Mega.
SD Card tends to be pin 4 for SS.
Note that you can often configure the SS pins in the libraries though, so you could check if they state D10 and D4 and leave them as is. You'd just need to get the 3 SPI pins to the correct Mega ones then.