Hey, welcome aboard with a great drawing. A few comments though;

I'm not sure the raked wing tips on the -8 would look as you've drawn them. Images I've seen seem to show it with more dihedral.

All your lower deck doors have rain gutters/bumpers for jetways yet the doors over the wings on the main deck should not have them if I remember correctly as no aerobridge will ever dock with them. I'm not sure if the aft door has them or not but I would guess not.

Also I would try and define the control surfaces on the outer wings and tail plane more effectively. On the real jumbos you can quite easily see the elevators and outer ailerons from such an angle.

-I think You could try to work more on the shading in places like leading edges,
-it seems that there's no "underwing" shade on the fuselage/wing roots,
-I have some doubts about the shape of the engine pylons - inner is weirdly bent "upwards" in the rear part, while the outer looks like the wing profile was completely flat on the bottom side.