Flat tappet cam. did you use the zink additive mixed with the oil on start up and run at proper speed for a length of time? New lifters on an old cam can wipe out the cam lobes, and if it's getting louder, that sounds like the problem.
Noise that goes away when the clutch is depressed in most times the transmission input shaft bearing. The input shaft should only have a very small amount of movement if the bearing is good.
To tell, turn the manifold upside down, measure where the manifold rests on the cork gasket, if 5 inches, it's 289, 302, if 7 inches it's 351-W, 351 C engines have a dry intake.