Love it, would it still be road legal for 7 digits 3+4 in two rows one above the other?
That depends whether the car is US or Euro. All Euro cars need full size characters to be fully legal and you probably wouldn't be able to fit two rows with correct spacing on a standard US mount.
If the car is a US import you can try to claim it doesn't have room for a full size front plate and therefore try to rely on the exemption that allows motorcycle-size plates and characters to be used on a car. But this is really only intended to cover cars where there's an "aperture" in e.g. the tailgate or rear bumper in which the number plate needs to fit. You see quite a few Japanese imports like this.
IMO the best option is to buy a "short" number like Jim W that will fit the US mount with full size, UK legal characters. My preference is to use a standard height UK plate with the ends cut off, so there are black bars above the plate and below the plate on that US mount. It's a bit fiddly to organise but gives a good result.
C8s won't need MOT test for a while. However, latest MOT rules include,
* Registration plate characters must be the correct size, stroke width and spacing. (This includes spacing around the edges.)
* Registration plates may be in single or two line formats.