You may not like my advice, but here goes. If as you say you have high grade pure lead, then it is very valuable. Straight WW would be an excellent choice for the loads you are speaking of. You could easily trade or sell the good lead for a premium. How much did you pay for it?
200 pounds is not alot. You may be able to trade for a lot more WW. I do not know where you are located, but I would gladly trade 400# of clean WW for 200# of pure lead.
You do not need to mix alot if you keep your velocities at 1200 fps. At that velocity I do not believe you would have any trouble if you cast the pure lead without adding any of your harder lead. I would just mix in 5% tin to give a good mold fill, and not even use the other lead you have(since you are not really sure about what it is). If it is ringing when you drop it, it could easily be WW. Tom.