Goos morning Shadow. With most Deans kits a little 'tweaking' is required with templates and plastic parts. best to cut everything out over size and then fettle them to fit, it tells you this on the general instructions page in your kit instructions. Just remember to shape the decks evenly on each side, otherwise the openings for the superstructures will be off centre. Have a look at Deans Marines forum, there are several build logs of their warship kits, not much on Cossack, but plenty on similar kits, all seem to encounter similar problems to yourself. The HMS Solebay being a case in point.
http://www.deansmarine.co.uk/index.php?topic=427.0 as the bloke building it says, Deans kits are 'modelers kits', with most of the printed plastic parts needing adjustment to obtain a good fit. The HMS Kelly build thread also flags up similar problems
http://www.deansmarine.co.uk/index.php?topic=227.0. Also, the plans will vary in size slightly depending on temperature and humidity, for example, on a warm, dry day, a part may measure out at say, 150mm, on a cold damp day, the same part will measure out at say, 152mm. So basically, you will have to make compromises as your build progresses.....