The stock length for a 409 rod is 6.09.and it's an old trick to use that length rod on a 409 crank[3.5]and a 348 piston,however I'd suggest having a piston built for the application,or you can save considerable money by using the K-B piston for the 4 inch stroke,a 6.385 rod[same money as the 6.135 rod],and a 3.5 inch stroke[409 crank].Y our math should be 9.600[blueprint spec] minus the piston pin heigth,1/2 the stroke length,and the rod length that you intend to use.A 409 crank,an I beam 6.385 bushed rod,and the K-B piston[642 grams plus 142 pin weight] will need very little work[no mallory metal] to bring it into balance.I agree on the price on those 333's.I'd suggest a little bowl work such as opening up the throat under the valves to 85-90 percent of seat diameter,blend the machining marks from the machine work into the bowl area,and a port match for your needs and goals.The K-B piston,6.385 rod,333 head will put you at around 10-1,and depending on how you cam it,will make plenty of power on pump gas.