/6 random thrown rod

^^^ Yep, I was gonna say... if you have another engine of the same year series (up to '76 or so, or afterwards when they changed to the cast crank), then maybe you will be able to swap in another rod and piston from the spare. You gotta pull the head to R&R the rod and piston, and do some other stuff like remove the ring ridge, but....

And you could even drop out one crank and put in the other; or just change all the bearings from underneath. You can get fresh bearing sets pretty cheap on eBay.

Find out if Iffy Lube messed up and then go from there. What is wrong with the spare engine? When the present knocked, was it one single knock at a steady repetitive rate, or was it a collection and clatter of different knocks? If one, then probably it is just one bearing.

Any oil leaking out anywhere?