Yes, even here, in Russia, it is hard to find many information about there engines.
Most part of sources stated, that GAZ-562 diesel is the licence copy of Steyr diesel engine.
I have found the following data about GAZ-5621 version (the dimensions are given for engine with clutch house and without gear-box):
- volume - 3,2 ltr;
- piston diameter/stroke - 128/175 millimeters;
- length - 973 millimeters;
- width - 603 millimeters;
- height - 809 millimeters;
- weight - 310 kilograms;
- number of cylinders - 6, in line (not V6);
- max. power - 81 kW/110,2 HP on 3500 rpm;
- max. torque - 460 N x meter on 1800 rpm;
- fuel consumption - 210 grams on one kW-hour;
- toxic norm - EURO II with neutralizer.
Here are images of:
1) GAZ-5621 engine:
2,3) GAZ-562 engines:
Sadly, I have found nothing about SMD-21-08 engines, besides, they are used in some armoured vehicles.