Anyone know anything about flathead 6 engines?

The displacements of the Mopar L-head 6 were 216, 230 and 251 CID if I recall correctly. The design goes back before WWII. They were well built and gave good service so long as they weren't expected to do something they were never designed to do. They had a long stroke and 4-main cranks, and therefore were not high RPM pieces (but back then, there were no freeways and many county roads were still gravel so highway speeds were much lower).

As for the knock, it is most likely a rod bearing. You can probably just replace the inserts and get some more life out of it. If you are intent on a V8 swap and/or just like destroying stuff, keep running it.