To keep it relatively simple, I discovered that Smart Render and LambdaCraft are incompatible with each other on my 1.7.10 version of MC. If both mods are loaded, the game will crash when going into 3rd person.
After consulting r/FeedTheBeastCrashes, I now know that both mods edit how the third person model is rendered (or something like that), and was told that Lambdacraft *could* be edited to be compatible with Smart Render.
Except... I kinda have no idea what to do. I have little to no experience with making/editing mods, so I dunno where to even start.
If anyone is willing to point me in the right direction, even if it's something as simple as linking a wiki article, I'd really appreciate it.