You would need to tool up a new Trailer Second, as well as new underframes for the Driving cars, as these were DC only. The good news is that you could then also use the TS with the 314 to make a 315. You could also model the 508 in original 4-car configuration for the South-Western lines, as well as Merseyside, pre-refurbishment.
Depending on costs, could you just get a new end slide done for the post-refurbishment 507/508s - there are some wonderful liveries you could do.
At the end of the day, it is making the numbers work. I am sure there are a lot of NW modellers who would be interested but whether that would be enough to make the project viable is another thing.