This Dakka article covers the common scaling issues pretty well. As mentioned, the main difficulty in translating real vehicles to
40k is the heroic scaling of the game's models - vehicles with the proper overall dimensions will have comparatively tiny details (wheels, doors, etc.). It can also be difficult, in the first place (although you seem to have that covered), to find vehicles that roughly match their
40k counterparts in
all three dimensions (tanks with the proper footprint will be too short, trucks of proper length and height will be skinny, etc.).
With sufficient conversion, though, historical kits can serve as wargaming models just fine. Plenty of people have done it before and not
just for Orks, although it becomes easier to mask (or
translate, if you prefer) the original kit when ramshackle add-ons are expected.