But...You did mention oil pressures that are low. I don't think I'd go through new lifters unless you can figure out why the pressures are low.
Before sticking a bunch of money in it only to find out it has a more serious problem, try some more diagnostics.
What is the compression of each cylinder? Try a leak down test too. That will help determine whether or not your valves or rings are shot. Make sure the engine is timed correctly and note the spark plug colors. If you have access to a scope that you can look into the cylinders with, look for carbon, or lack of carbon on top of the pistons. A piston that is really clean, or the edges of the piston are clean (bare metal) would suggest that oil is getting past the rings and "washing" the top of the piston. The top of the piston normally should be carbon coated to some extent, and colors ranging from tan to brown to black depending on how well the fuel is mixed and burning.
A significant amount of "wash" around the valves can suggest that valve guides and/or seals are worn and have accessive clearance.
Those things won't tell you why the pressure is low, but it may tell you whether or not your engine is just tired and worn. You may have to pull the oil pan to find out what kind of shape your cam, main and rod bearings are in. You could also use a long feeler guage to get an idea of how worn your cylinders are.
I think your lifters may be making noise because of low oil pressures, probably caused by excessive main bearing clearance. But that is just a gueass. Without verifying a lot of things first, it's always a guess.
