Newbie motor questions

[Googenheimer]

My first post here. I just purchased a '67 longbed that is in really good condition for the price I paid. It's a great truck but I have some questions about the motor that's in it. So far I've found out that it should either be a 283 or 327 (if it's even original). Is there a way that I can easily identify it? Also the motor runs well and only uses a little oil due to leaks, but it does have a tinny sort of knocking sound almost like someone was tapping the bottom of a tin can that corresponds with the RPMs. Then when I accelerate it sounds like a diesel until the RPMs get a little higher then it goes away. Obviously something is wrong, I'm just wondering what it could be. I am hoping it may be carbon deposits causing preignition or something and that is where I will start..but that is wishful thinking.
Also, if I do have to get a new motor, what would be the easiest V-8 swap that will mate up to the manual 4 speed? Thanks!