Well, the two powders are not really all that close. IMR 4064 isan extruded powder for medium size cases; and IMR 4350 is a slower extruded powder for larger capacity cases. The latter is probably one of the slowest powders one would consider for this application whereas IMR 4064 is about right down the middle of the road.

However, each gun has its own little preferences based on barrel harmonics and a bunch of other factors. You never really know what a given gun is going to like or not like until you go to the range. My particular 7MM-08 seems to like H-380 with several different bullet weights - and doesn't shoot very well with IMR 4350, at least so far. I could take that 25" Douglas barrel off, cut it to 22"; and it might shoot like nobody's business with IMR 4350. And then again it might shoot everything better . . . or everything worse. You just don't know.

Having said that, however, there are certain bullet/powder combinationsthat seem to shoot exceptionally well in a majority of guns of a given caliber. These are quite often the first load one would try in a new gun. But if it doesn't shoot with one of these "pet" loads, all is not lost. It may drive tacks with some other combination that shot really mediocre out of the last ten guns in that caliber you tested.