4 mids or 4x6 plates?

I have a 1968 Mustang and I am in the process of customizing the audio system.  I am starting by replacing the speakers at the factory locations.  One of these locations is the lower half of the inner door panel, but only in a 'deluxe' edition car.  My car has grilles made for the entire length of the door, which have undercut areas for the speakers and for the courtesy lights.

The size of the original speaker that ford installed was 4"x8".  My car is not 'deluxe' yet, as I am in the process of upgrading the interior options and what not.  So my quesion is whether to cut for a 6"x8" speaker (but I think I can't find a speaker like that), cut for a 4"x6" plate speaker (probably with the tweeter and woofer made separate), or cut for a 4" midrange (some sort of separate or component)?

What will produce better quality sound?  What will produce better imaging?  Assume I have applied all sound damping material necessary.

Thanks for any and all suggestions.

ADAM D FLOURNOY

The original speaker size was 4x8, not 4 or 4x6 or something standard.  I can't find anything that measures 4x8, but I know I can use 4x6, or 2 4" speakers side by side.  I only have a bit over 4 inches of clearance top to bottom of the mount, but I have just over 8 inches of clearance front to back.

Tweeters were certainly never used back in 68, but I might be able to find a place in the door for one.  I will not surface mount a tweeter anywhere, but flush mounting is an option.

Let's say I can't find a place to use a tweeter.  I already have a pair of quality mid-high custom speakers in the dash, so do you think a 4x6 plate with separates, or just the 4" mid from a component?

ADAM