I don't really think any of the lee bullets are much good to 400 yds, the 500 3r does alright to there, but craps out beyond that especially if theres some wind. The blocks are pretty lightly constructed for those big bullets and don't hold up very well, and whether or not the set you order to replace the first set will be the same diameter is anybodies guess.One of their 405 gr versions might be alright, but they'll have troubles past 600 yds.
The Saeco 645 mould throws a 480 gr bullet that works rather well to 1000 yds and a bit further , plus is very accurate at close ranges. While I haven't shot that bullet with smokeless I would expect that at the standard velocity smokeless loadings it will do just as well as with black.
Most folks make the mistake of going with a bullet that is to heavy for the 45-70. The heavier bullets over 500 gr are better left to bigger cases, and even then if you look to see what the old deadguys used, they seldom if ever loaded a bullet over 500 grs unless the powder charge was in excess of 110 grs.