You are going to use the 350 balencer and flexplate and have a machine shop balence it.I would think that in a blazer you would go the other way 400 crank in 350 block for the lower rpm torque(383 cid) the engine you are building is a much higher rpm engine(377cid)and would probably be much better suited for a camaro or nova
The 400 sbc with the 350 crank is a good combination, this is a destroked 400. The stock 400 engine uses a 4.125" bore x 3.75"stroke. The 350 crank is 3.48" inch stroke. In order to fit the the 2.45" inch main journal 350 crank into the 400 block a bearing spacer is used because the stock 400 crank is 2.65" inch diameter. (the rod journal diameter is the same). Don't worry,I have never seen these bearing spacers fail. Since the 350 crank is used, you must re-balance the engine because of the heavier weight of the larger bore 400 pistons, however the use of the 350 crank now makes this engine "internally balanced", therefore, you use the stock 350 flex plate and harmonic balancer. These engines are a good street rod engine in a light car. They are quick and high winding (lots of rpm). GOOD LUCK on your project.