No there is no problem to do a little extra countersinking - as long as you are VERY careful. On a 2mm chassis, it's difficult, as the official (from an engineering point of view ) countersinking depth for an M3 screw is 2mm!! If some of the screws happen to have a slightly thicker head (i've seen this) then it makes it an even bigger problem.
The front belt shouldn't be too loose - remembering there is belt tensioning on the front diff mounts. It shouldn't be possible to fit a belt that is 4 teeth shorter - that's 12mm!! 171 is the correct length belt.