A simple crank and piston swap will turn a 230 into a 250, and a 250 crank is fully counterweighted. Pistons for a Chevy 307 V-8 have the same bore, almost the same compression height, and are flat-tops (stock 250's have dished tops). Read Leo Santucci's book on hot-rodding the later Chevy six. Also, check out www.cliffordperformance.net
, and www.sissellsautomotive.com
Inline sixes have problems with torsional twisting of the crank, and they tend to want to shake flywheels and balancers loose. Be sure yours are attached well.