Looks like the cartridges are 45-70 ? The best accuracy I've heard of with an LBT mold from a Sharps 45-70 was 1/4 inch with 10 shots at 100 yards,this with a scoped Pedersoli, and quite a few long range competitors have reported an inch and less at 200 yards with open sights. In other words you have room for improvement, and it will come easy when you understand the technical facts, which I'll explain after the following paragraph, which is to answer your question about sizing, based on your present technical understanding..
Minimal sizing does no damage to a bullet, and is in fact is mandatory to obtain a precision fit with every round. If the mold is cut to cast at groove diameter or slightly over, the straight wall cartridges used in Sharps rifles will chamber every bullet easily without sizing, and will shoot better with unsized than with sized, because it fills the chamber better.
For the ultimate accuracy for all Sharps rifles, and this includes most modern bottle neck cartridges to some degree, but not entirely, these are my recommendations:
!. Lap the barrel using the LBT bore lap kit. This can be done using bullets from the mold that will be purchased. LBT push through slugs will show you where any variations in the bore are, their severity, and when the bore is made perfect by lapping.
2.Determine maximum bullet diameter that will chamber in the gun of interest. A simple way to measure is to measure the cartridge neck of a loaded round with whatever bullet you are now shooting, then after firing, measure the neck again. Whatever the difference is, add to diameter of the bullet which was just fired and order your mold that size. This information is specific to the straight wall cartridges used is Sharps rifles, and can result in diameters as large as 464 with some of the older rifles, but with the Sharps of modern manufacture, diameter is more likely to be .458 to .460. The customer who's 1/4 inch accuracy is named above followed my prescription to this point, and required a .460 diameter bullet. He chose a 450 gr spitzer gas checked bullet and was shooting smokeless powder.
3. If any variation in neck wall thickness can me measured, neck turning the brass just enough to make it uniform will improve accuracy, and quite dramatically if variation is more than .001. This and the close neck to chamber fit are to minimize bullet tip before takeoff. If bullet diameter is more than .002 larger than groove diameter, I often cut the first driving band at groove diameter, and especially if the rifle has a longer than normal throat. Most have only an abrupt taper into the rifling a short distance forward of the case.
You asked if wheel weight metal is a good alloy. Yes, and the above customer used them, water quenched from the mold. Castibility will be best if 1% silver bearing no lead plumbers solder is added. 2% will make it cast even better, but don't exceed that amount or age softening will become a problem. Linotype or a commercial casting alloy will cast a little bit easier, by which I mean sharper and more uniform fill out in the corners of the driving bands, which is the only way rich alloys help accuracy. Quenching them will insure maximum hardness with whatever alloy is used, but it won't exceed that of quenched WW alloy.
Tin lead doesn't harden when quenched, nor do I recommend using tin lead alloys with LBT design bullets. If one selects a bullet design similar to the postel, with a long bore riding nose that is undersize, which is common for black powder shooters to use, tin lead alloys are mandatory, in a hardness which allows the bullet to slump to a complete groove diameter fill all they way to the beginning of the round nose. Bullet softness is regulated by accuracy, and if one goes to the effort of recovering spent bullets, they will find that accuracy always peaks just as the above obturation is obtained. So if this is the type bullet you are using, use a soft alloy, not quench hardened WW alloy. And don't expect top accuracy. There is no way an undersized bullet can be slumped to a perfect bore fill and produce the accuracy of a hard bullet that already fits and doesn't obturate to shape.
Most black powder competition demands plain base bullets, but specify that you will be using black powder when the mold is ordered, as I cut special lube grooves for black powder plainbase bullets, to minimize fouling from both black powder and leading.. An LBT design bullet fitted as above will still give the best possible accuracy, with plain base black powder shooting, but will often take up more powder space than most long range shooters can put up with, if shooting 500 gr bullets at ranges over 300 yards, as velocity will be too low. Those who will rethroat their rifle so the bullet can be seated out far enough to allow the desired powder capacity will get best accuracy, but such a setup can require cleaning of the throat every couple shots in hot dry weather, as fouling will prevent the rounds from chambering.
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