I am the supevisor ao an Diagnostic Imaging dept. I have been involved with the planning of 2 complete x-ray departments. The lead is just plain soft lead.
Get all you can. You might get it free for tearing it out. The only problem I see is when you melt it you may have a lot of junk on the top from the plasterboard and paper glued to it.
In general, wheel weights are harder than magnum shot. OF course, not all wheel weights are the same. Mixing pure lead (soft) with something like 20% soft lead should produce a good mixture. A mixture of 10% soft lead + wheel weights is also good. Because the hardness of the wheel weights is unknown, conservative judgment is the best guide.
This information was taken from a Lyman reloading manual. Wheelweights are 95.5 lead, .5 tin, and 4% antimony. The hardness is 9 BNH. Pure lead is 5 BNH Taracorp magnum shot is 92 lead, 2 tin, and 6% antimony with a hardness of 15 BNH. Linotype has 12% antimony. Our magnum shot that we shoot is much harder then wheelweight alloy, but wheelweight cast bullets can be heat treated to exceed the hardness of magnum shot the way we buy it.
pure lead is still harder than the target. I have run a 50-50 mix WW to lead If you keep it hot does ok but will run about one size larger shot.Adding pure antimony works If you use an alloy flux to start with. It will not blend with beewax rick