I'm a bit surprised by the comparison of 365s and 350s.
From my experience 365s and 465s are virtually the same but one is AC and the other is DC. I think both types are a bit like a 323 but with a bit extra added on the spec i.e. 4th carriage, FC section and a bit more of a streamlined exterior.
The only unit I've been on which I feel is like a 350 is a 185 which is obviously the diesel version of the same thing.
Going back to something else said in the original post I find it surprising we keep seeing new train designs. Once the Turbostar had been built we just needed to get modified versions to suit different services, like with the Sprinters. A 110-125mph Turbostar would have been much better for TPE than the 185s yet all the work goes in to the Desiro diesel and then it's found it's more fuel thirsty than other units and has weight problems on one of the lines it was supposed to be running on (Selby-Hull.)
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They would probably have to stick to former Great Western lines, which suggests Valley Lines or the Bristol area. All the Pacers would be displaced, and might go to reinforce Northern services.
Northern have got a headache to decide which services get downgraded to Pacers when the LO 172s releases the 150s to FGW which releases the 142s to Northern and East Coast get their 180s back.
The problem is not every line is cleared for Pacers. Pacers are less good in areas where there are high gradients and has been subsidence due to mining in the areas, so even on some lines where they aren't banned they're limited to 20mph when the Sprinter speed is or could be 50mph. There are also some platform issues at certain stations.
It's also not as simple as saying you'll have more units so you can use 2 Pacers to replace a 156 or 3 Pacers to replace 2x150s as doing that on some lines would make the train is longer than the platforms and SDO is not available on Pacers or most of Northern's other units.
Northern need new 23m carriage trains not more Pacers.