On the cast welding subject - preheating helps get the penetration but post heating is more important. This is because the cast cools faster than the mild steel and will hairline crack at the weld. Sometimes even just creating enough stress between the mild and cast that it will crack sooner than later.
If it's small enough, put the recently welded object into a bucket of sand so it will cool evenly. I've also just ran a torch across the cast side of the weld every minute or so until it seems to be cooling as slow as the mild side. Pain in the butt for sure, but it's cheap insurance and peace of mind.
