The cross correlation between the observed series is of little value . You should use single pre-whitening to aid the initial identification. Make sure you account for anomalies in the individual x's before you form the arima filter
use pre-whitening
make each variable stationary and use the filter for each of the x's separaetely to estimate the ccf
no need to use Granger here ,, you are trying to identify an initial model
absolutely no detrending is needed to create the arima filters unless they are needed in concert with the arima model