There is only one receiver for the 742. The difference is as you assumed, the mag and the barrel. I would say though that the difference between the 308 and 30-06 is nearly non-existant. For sure not enough to warrant a conversion. Also it is highly likely that it would be ecconomically more sensible to simply swap.. 742's are common and the 30-06 is likely the most common of all. Why spend a couple of hundred to rebarrel, just trade for a 30-06. Even better, for the price a rebarrel is likely to cost simply buy a good used 742 in the 30-06! Good luck!