Got a '97 S10 with blown HG on a 4.3. Can a 4.3 from an '03 Silverado fit this properly and if so, are there mount modifications or other considerations I need before considering making the purchase of this used engine?

First question, how do you know it has a blown head gasket?

The blocks are the same. Only thing will be different is some of the sensors. As a precautionary, always transfer over ALL over your 97 sensors. MAF, MAP, TPS, IAC, CAM, CRANK, KNOCK, IGNITION to the new engine.

Cylinders are full of coolant.... either gaskets or cracked block. Sometimes easier just to swap engines..... I will check to make sure it'* just not the intake gasket, but the "gut" feeling is..............

Take the plugs out and crank it over by hand. Then do a compression test. They are pretty tough and lower gaskets are way more common if it hasn't been run that way

Easier to swap engines? 4.3?'*? I beg to differ. You'd be better off pulling the heads and replacing the gaskets vs swapping engines. And who knows WHAT you'll end up with as far as a donor engine.