xwpzxpunisher:
the o.g. bigfoots were running with a c6 so they cant be that bad

That was mostly what they had to work with and efficiency and long life was not a issue either nor was wallet when they build those things.  

After its said and done, the NP435 is going to be your cheaper, stronger and better tranny. Manual trannies are actually pretty easy to rebuild. They are just gears. Put the new gears in the right order and the only problem you might have is sealing your gaskets. And don't worry about cracking the case, your 300 won't hurt it at all.  

You are correct on that and forgot to mention it. They also run the heavy case c-6. Which I also run myself. Helps in case deflection, also run a really nice aluminum pan which is stiffer than the factory pan. Helps keep everything straight. If you get deflection in the tranny case the valve body sometimes leak and create lots of problems. Destroy's converters and thats expensive.