84 will be a 2 piece unless the motor has been swapped to a late 85/86 block.
I have done it. Its fairly straight forward in a k truck, with the c im thinking you have to lift the engine out of the mounts to drop the oil pan if I remember correctly.
Once you have the oil pan and transmission inspection cover off you remove the rear main bearing cap and oil pump pull out the old seal with out nicking any thing and replace with the new one and reinstall the main cap, Oil pump, reinstall the oil pan(I like the felpro blue rubber one piece perma gasket), reinstall the inspection cover and reinstall engine mount bolts.
There is a little more to it then that but that gives you a ball park idea of what your in for.
Double check that the oil sender is not leaking before starting as I have seen guys think their rear seal was leaking only to find out later that it was actually the oil sending unit that was leaking