[CanadianCamaro]

You can have it reinforced and plastic welded - but it will be damn near impossible to get a perfectly smooth finish on it.
Yes the whole front nose is 1 piece.
Yes the V8 fits on the V6 nose. But as someone said the grills are separate part numbers.
Height difference....The shape of the V8 bumper is different - you probably know that already and others have said so. But let's focus on the bottom/underside. The lower lead "point" of the V6 bumper (i.e. the most forward point of the car) is slightly higher than the same spot on the V8. But I think this is mainly b/c the underside of the V6 bumper (the pained part between that point and the air dam) is on a greater slope than the V8 which is close to parallel to the ground (not quite but close). One would think that this would then mean the bottom of the air dam is further from the ground on the V8 - but this is not true b/c the V8 sits slightly lower than the V6. So the gap between the ground and the air dam is pretty much the same on both cars. So as you can see, the answer to your one question is.... complicated.