I there a ridge on top of the bore? That usually tells you the extent of the cylinder condition. Does it smoke? Is the compression bad? If the cylinders are good (lots of magnums dont even show a ridge and can still see hatch marks, hard blocks!) you could just do a ball hone glaze break, rering and bearings. Could be another 100K out of it. Depends on the cylinder wear. Same on the crank: pull it and look at the bearing journals: scratched enough to catch a fingernail? Bearings down to the copper? Crank end play too much? Many times cranks are good enough to just install new bearings or cut them .010 and run new undersized bearings. Its really up to you and the condition of the motor as to know if an oversize/undersize rebuild is mandatory or just a bearing and rings rebuild. Definately get a valve job, they will get those heads looking new and address and valve sink.