It can be done by replacing the original 30/30 barrel with one in .35 caliber; and also, since the 35 Rem case head is smaller than the 30/30, one would be required to replace the bolt. All other parts are the same; but given the fact that 336 Marlins in 35 Rem are common as dirt, such a conversion would make no sense from a economic standpoint. But if this guy insists on the conversion, have him send the gun back to Marlin; Marlin performs these kinds of modifications on a regular basis.