6v6gt: It is not in the data sheet. The most concrete information about shared use of SPI lines, that I can find, is here: http://ww1.microchip.com/downloads/en/appnotes/atmel-2521-avr-hardware-design-considerations_applicationnote_avr042.pdf , although this may not exactly cover your scenario.
However, there is a also lot of anecdotal evidence about the wisdom of keeping clear of the SPI lines when possible to minimise the risk of interference with attempts to program the device, for example: https://www.avrfreaks.net/forum/can-i-reuse-isp-pins-other-taskstarget-atmega164p?skey=reuse%20of%20spi%20lines
As a result of my own experience, I attempt to keep at least 1K away from the SPI lines where possible. I documented some of this here: https://forum.arduino.cc/index.php?topic=673817.0 (post#6 onwards)
If you won't be changing the bootloader, already have one then connecting SD won't stop you from changing your sketch.
With 328pb you have an extra UART capable of full speed master-mode SPI, Nick Gammon has the software to do it. You could have Serial RX/TX, ISCP and still have hardware-SPI. Atmel had really good engineers.