Testing is about the only way to find out. With the addition of 'hard' lead shot, which contains trace amounts of arsnic, you can heat-treat the bullets, perhaps up to brinel 22 (new lino) or even 30. I don't know how the pewter will react, but the shot will be necessary before heat treating can be performed successfully.
I've about 200 pounds of Wheel Weights left from a rmpant collection at tire stores back in the 70's. No zinc, and no pure lead stick-ons in the batch. It will heat-treat up to brinel 33/34 - 50% harder but more ductile than Linotype.