It takes a lot of heat to soften springs that size. As long as you’re not continuously welding close to the spring for a long time you won’t get enough heat transfer to hurt them.

Also, I would weld the keeper plate on, not bolt it. Bolts can come undone, especially with a nasty coyote chewing on it. Sure, in ten or fifteen years you might have to take a grinder and cut the weld to replace the springs but it hardly takes longer to do that than it does to remove bolts. Welds come off quite easy with a Zip disk on a dewalt

Break out the popcorn. The old center vs side swiveled debate. There's an old thread on here where a good arguement is made for a just off center swivel.

https://trapperman.com/forum/ubbthr...-do-not-want-to-center-swivel-your-traps

Long read but worth it.

That won't happen no matter where you attach your chain. It's all about the swivels when It comes to damage.