Raimey,
The discussion has been going on a long time and there were so many posts saying to use 70 grain bullets because that is what was available back then. My concern about using heavier bullets (or modern sleek, ones) is the possibility of them being too long for the twist rate. The concern about solid copper bullets is the same, plus the possibility of driving the pressure up. I remember you don't have a lathe, but I know someone that lathe turned .243" monolithic bullets to .228" before there were any .228 bullets commercially available and almost ruined his rifle in the process. The way you are sizing the cases is the right way to do it. If you full length size the cases, setting the shoulders back, will case separation after a few firings. The variation in velocity might be caused by variation in powder position in the cases. You might try positioning the powder the same way each time you shoot over the chronograph to see if that helps. Even though I use a filler in other cartridges, I caution against that practice with this cartridge. A slower powder might fill the case better, but I don't have any experience with VV powders. BTY, your description of the accuracy indicates the bullets you are using are not too long for the twist rate. Good luck.
Mike