Before you commit to a clipped wing Spit for Johnson's kite, check which one!
The most famous one of his, often seen in books and modelled by more people than probably any other Spit, was an early MkIX, with full-span wings. I'd need to check the serial number to give exact info though, which aint a problem. The one in question bore his initials as the code letters :- JE - J, had very small letters and numbers for the serial, and had a maple leaf in a white disc just forward of the cockpit. This was when he was leader of a Canadian Wing. As far as I know (but I could be wrong), Johnson never flew a clipped-wing Spit with these markings.
The 'pointy' rudder was as a result of making the chord (the width) broader, in order to provide more rudder response and, to an extent, counteract increased torque after the fitting of more powerful engines; the fin remained the same as the MkI on. This was the first stage of a redesign of the entire empenage, which led eventually to the much broader fin and rudder of the MkXIV and later models, as shown in the pic posted by Eric. From the Mk21 on, and on the Seafires from the same production series, the fin and rudder were changed yet again, being not only broader, but much taller also, and the eliptical wing shape virtually disappeared, attaining a straighter profile, not unlike that of the Tempest.
Again, on the cannon bulges, the 'blisters' over the breech and feed mechanisms, you need to know exactly which Mark, and which wing type. For example MkIXB, C, E, the reason being that, depending on the wing-type, and the armament fitted, the cannon blisters would be a different shape and size! These aren't easy to scratch build, even in 1/32nd scale, the best way being to either mould new ones in plastic card by the heat method, trim them and add to new panels, or try to mould them onto the wing in 'Milliput' or similar epoxy putty.
Never heard of a hole, hexagonal or otherwise, in the seat of a Spit. I've sat in, and taxied, a number of Spits over the years, MkII to Mk19, and all of them, even a MkII that fought in the BoB, had a 'solid' metal seat. There is often mention of, and modelled, a 'bakelite' or 'plastic' seat, dark orangish red in colour, but again, I haven't seen evidence of this, although I assume such a seat on some Spits must have existed. The one in the MkII mentioned was the original seat by the way, just in case somebody argues that it would have had the bakelite seat and must had had a replacement fitted!
Hope this has helped you a bit mate, good luck!