I dont know anything about babbit, but if the bullets are that hard, I agree that softening them may help.
I would try adding some pure lead to a batch of maybe 50-75 bullets and see what happens before changing a whole bunch of alloy. If its only a little bit of fouling in the forcing cone and the barrel is clean, I wouldnt worry much about it myself. If its working, keep shooting.
You can add stick on wheel weights or lead pipe to soften it and if its as hard as LE-28 thinks, I would try 50%. Did you slug the barrel? It could be you need a hair bigger diameter bullet.