Crimea_River
Marshal
A very large gap. My first thought was to try to force the fuselage wider with a piece of sprue cut to size and glued inside but it looks like you have the cockpit glued in which will prevent movement. All you have left is to fill the gap and I would suggest gluing in pieces of styrene sheet and then shaping the material with a file and sandpaper. I think the gap is too big for putty alone.