Before you go any further I suggest you slug your bore to find out what size it is, there are plenty of tutorials on how to slug you bore out there. Marlin seem to make it a habit to make their barrels oversize. This is what has given rise to the myth that micro groove barrels wont shoot cast bullets. My Marlin 45-70 was .4588", seeing as my mould cast out at .460" I either shot the bullets as is with a dip lube or used a .460" sizing die.100 yard groups under 2" were common, which I didn't thing was too bad considering I was using only a 4X scope and no sling. I slugged two other Marlin .45-70s and a quite a few .44 and .357 magnum rifles and they were all at least one and a half thou over size. Cast bullets need to be at groove diameter or larger to shoot accurately.
Try other alloys, pure lead shrinks more than alloys, when I played with making moulds I found that whilst with alloy I could cut the cavity to the bullet size required, pure lead needed the cavity to be at least .003" larger for a bullet of .45 diameter. Also, try casting hotter, particularly with ensuring a hot mould and a good puddle of metal on the spru plate. Forget about trying to open up a mould with a dremel, unless you have it set up in a lathe to control the cutting, in which case you are better off using a profiled boring bar or spoon cutter. Instead try lapping it out with grinding paste. For instructions on working on Lee moulds look up "Leementing" at the castboolits forum