P W B Semmens "Electrifying the East Coast Route" (pub 1991) refers to a BR ER and NER "Report on the Modernisation and Electrification of the East Coast Main Line between King's Cross and Newcastle and Certain Associated Lines" of June 1959, also a 1957 BTC Electrification Committee Report. From which he states that express EMUs (i.e. 309 type) would be used for at least some services - including to Leeds and Bradford; and also quotes some details of the proposed timetable.
I think he wrote an article for Modern Railways at that time (c1990) on the late 1950s' proposals, which had a map showing what was then proposed - which (IIRC) included to Sheffield (which would been seved from KX, not St Pancras), also for (for freight to Colwick).
It was different sort of railway then! One difference was the amount of regional autonomy, which is why the ER (and NER) could opt for express EMUs, while the LMR chose to make everything loco hauled.
If you read Terry Gorvish's official
British Railways 1948-1973 : A Business History it becomes painfully obvious that there were loads of very dodgy 'official' documents bouncing around that included some very poorly developed, incomplete, flimsy and frankly 'guesswork' Business Cases behind many supposed Projects. These tended to gloss over awkward bits such as conversion to electric train heating, alterations to stations and costs of disruption during the inevitably lengthy construction programmes; let alone more financial implications such as interest, discounting, rates of return and so on.
After much to-ing and fro-ing between the BTC and the Ministry, various Select Committees and the Stedeford Committee it wasn't until Ernest Marples was at last able to formally give the 'full' WCML scheme the go ahead on 30 January 1961. The first shovels had gone into the ground at the start of 1957 and the first electric runs on the Styal line were in 1958. The inaugural locomotive-hauled train from Manchester Piccadilly to Crewe had finally run in October 1960, I believe, but this was really only preliminary and experimental work before the full scheme.
The idea that there was any serious or detailed 'parallel' ECML electrification scheme being developed in line with the WCML is completely laughable, let alone specific rolling stock design choices.
It is, however, fair to say that the initial
EMU requirements for the WCML scheme (AM4s) were developed alongside the Eastern Region schemes for the LTS and 'Chenford' groups (and, indeed, the Glasgow 'Blue Train' AM3s). This was for a common platform with 6.25kV/25kV capability, 75mph top speed and multiple control arrangements, etc.. This was obviously helpful for the supply industry. This did
not include later the Clacton AM9 units