Do you know that the new one is the same as the old one? The 20 degrees hotter might just be what it's rated at, or your old one might not have been working. But we need more details. Twenty degrees hotter than what? What does it read on the gauge now?
Never seen one split like you have. A hole rusting through, sure. But a split seam? Nope.
The plug in the top was standard Ford stuff on certain years, and many of the replacements have them. Don't ever tighten something like that down as tight as you can though. It's a tapered pipe thread and, if you're strong enough, will simply crack the new casting from overtightening. Tight? Yes. But tight as you can? Nope
It might be in there perfectly tight right now though too. So just check it initially to make sure it's not just finger tight, then run it as-is until you see a leak. If it leaks, tighten it. If not, leave it.
I like to use thread sealer, but some recommend against it. Whatever your choice is then, that gets it to stop leaking.
Paul