The rule of thumb on such engine noises is a heavy knock, knock, knock is a bad bottom end, usually the big end of a connecting rod, rarely a main bearing. A sharp fast purring tapping noise is a wrist pin, this can also be a bad lifter, which is more of a clicking sound. to tell which, get a engine stethoscope or a wooden dowel and listen to where the noise is coming from.
I don't know what you want to do, You could rebuild what you have and save some money. American push rod engines are real easy to rebuild. get a rebuild kit and have the necessary machine work done and assemble the beastie, or have the machine shop do it for you. With the gigantic number of used 302's, and 351W's out there a used engine is definitely something to consider, or you could just get a long block and be done with it.
My Carbon foot print is 57 tons per year, and that doesn't count my fart gas.
I spend my carbon offset money on my 1995 F150 Flareside with a carbon spewing 5.0L V8.