I looked at "PSoC 4 Serial Communication Block (SCB) 4.0"
Number of SS
This parameter determines the number of SPI slave select lines.
Only one slave select line is available in Slave mode and it is not optional.
The values between 0 and 4 are valid choices in Master mode. The default number of lines is 1.
So, P6.4 and P6.5 are not #0 the below.
In the datasheet
Alternate Pin Functions
select0:0 for SCB3 seems to be P6.3
But in the Pinouts
There is no P6.3 in the 64-TQFP package (nor 44, 48).
So I'm afraid that you need to use SCB ~ SCB for SPI Slave in this device.
P.S. as far as I checked...
MOSI MISO SCLK SS
P4 P4 P4 P1 or P4
P2 P2 P2 P0 or P2
P5 P5 P5 P0 or P5