:-) Very nice looking cockpit, the fact that you used none Mk V PE set will not matter. Not many will be able to tell the difference anyway. If it was a Mk I cockpit you used there is very little difference to the Mk V.
Martin is correct about the wing strengthening ribs, the drawings I have do not show them. However I do have a photograph, in the same book showing them, so It might not be as "cut and dry" as it would first seem. Martins suggestion of finding a picture is the way to go, but that might be impossible to find. I am also building a Tamiya Mk V at the moment and unless I find proof, soon, I will be removing the ribs. To remove them I will use a new scalpel blade then clean up with fine emery paper, then re-instate the panel line. If you look closely you will see a small protrusion just inboard of the ribs. This represents the wheel up/down indicator. I will remove this as well, drill a 0.2mm hole and insert a short length of wire, painted red.
Drilling out the cannon barrels isn't difficult, if you do as suggested and mark the centre first. I use the sharpened point from an old pair of compasses. Best of luck :-)
Remember, "you just can't have to many Spitfires".
Mal