1965 Barracuda 273 rebuild...intake code?

Your rattle in the lower end is probably either a wrist pin with a loose bushing, or a rod cap that was getting ready to give up. On the intake bolt deal....look around on here and see if you can find a thread that was going about that a while back. Somebody posted about some 6 or 8 degree wedge washers they used under the intake bolt heads to even things up when using early 273 heads like you have, with a later intake. If you wrap a round or two of Teflon tape on the intake bolts starting about 3 threads in....you won't have oil creeping up the threads and ponding up in your intake. Good luck with your build....starting on my 273 build for my 65 Barracuda soon myself!