A scale kit should have an accurate outline, no two airplanes are identical inch to inch, But the kit should not radically change the outline, wing dihedral, engine placement, canopy shape from the true outline. The relative ratio between span and length should be same as the full size airplane. If the model is 1/8 scale the fuselage should be 1/8 of the full size and wing should be 1/8 size. All dimensions should be 1/8 of the full size airplane.

Several examples of things not to do (I have plans that have done this)

DC-3 pushed the engines outboard to gain prop clearance to the fuselage, yet did not change wingspan
B-29 push the wing back to improve CG location, nose was too long, tail was too short
enlarge elevator to improve stability yet wing and fuselage is normal size

When you read the rules for scale competition they judge several things:

1) scale outline
2) Markings
3) Color scheme

1) - don't change outline it sticks out like a sore thumb when compared to a 3-view
2) - don't change markings from full size, must match full size letter for letter
3) - Paint scheme must match full size being copied, color for color

Just my thoughts on this
Fred Cronenwett