When the service was split, from my own memories and observations it was mostly 170s on the Birmingham - Liverpool, until as you say the 350s were introduced. There were still I believe some 323 in the peaks as well, including at one point through running from the cross city line.
A Crewe based LNWR guard has already pointed out that only Crewe signed Preston when the LM service went there, and they never signed 323s, therefore unless piloting was involved 323s at Preston will have never happened.
I have heard of a possibly apocryphal tale from Central Trains days however, of the time a set swap between Birmingham and Nottingham direction sets took place at Liverpool - which caused something of a problem when the Nottingham crew went down to the Platform after their PNB to find a 323 waiting to bring them home.....