Have you tried 300*? I'd cure them for at least 30 min.... longer if it was me, prolly more like 45-1 hr.. It won't hurt anything.

I'm not even sure you'll get a good flash on the paint at 250*.

What possessed you to go lead free?... and what kind of alloy is melting at 350*??? If they were tin, it would take 450* for them to start melting. Do you have a thermometer in your oven?