Valve train noise is typically a higher clatter type sound. A deeper knocking sound is usually in the short block, typically crankshaft related. Unless your totally convinced of where the noise came from you might seriously consider unbolting the front motor mounts from the Kframe, jacking up the short block and pulling the oil pan.
Watch for things that bind as you raise the engine such as may have to remove distributor, radiator fan shroud,etc. You can lift the engine from above, or jack up on block of wood below oil pan. Once engine is lifted enough stuff blocks of 2x4, 4x4 or such between the mounts and kframe and let the engine rest back on its mounts...shake engine around a little to verify its not going anywhere.
With the oil pan off, then rotate the crank so a pair of rod bearings is at the lowest point and the rod caps to inspect the rod bearings. Check them one at a time, and put the CLEANED cap/bearing back on FACING the correct direction. Do every rod cap. Then do a couple of the main caps, though don't do #3 thrust or #5 rear oil seal. Usually the main bearings are ok even if the rod bearings are worn out. Detonation can wear out the rod bearings and all the engine needs is a new set of rod bearings to run for years more.
If theres a bad rod bearing but the cranks not damaged, just replace the rod bearings for all 8 rods. Plastic gage the crank with the new bearings, its usually all right. While the pan is off you could inspect the oil pump, though the original pump is usually in good condition. You can put together an engine that will last another 40k-50,000 miles and not have much money in it. The deep pocket people with $$$ may disagree with me but it can be done...I've done many a customer car this way. IN the 1970's the dealership I worked at ran overhaul specials all the time...piston rings, rod bearings, a valve job, and gasket set. We did hundreds of them and the customers on a budget loved it.
