The modification depends on the particular generation of Model 70. The pre-64's and post-64's, until the introduction of a dedicated short action receiver, the Model 70 used one receiver size, and accommodated different cartridge lengths(of the same case head size) by using various bolt stop lengths, and magazine blocks. With an unmodified 30-06 action on the early(or even more current) Model 70 generations you could most easily accommodate any of the 30-06 parent cased/receiver (ie 280/AI, 25-06, 6.5-06 etc), as well as the 284 Winchester based cartridges(ie 6.5x284N). With the pre-64 style actions/magazines, going to a shorter cased(ie.308,243, 7mm-08, etc) of the same bolt head size as the 30-06 parent case, would require the specified bolt stop, and magazine block which can be changed/installed.,..If you already have, or can find these parts