Intake manifold/lifters

[1971LT1SWBPOSI373]

I have a 96 Silverado with the 5.3 liter engine. I get the antifreeze smell when I shut down the engine so I suspect the intake manifold is leaking, and I constantly have to add antifreeze. I have not been driving it (for about 9 months due to a busy work schedule), as advised by a board member until I get it fixed. Well, I have the parts and positioned the truck getting it ready to tear into. The other day I started it up and a lifter was ticking loudly for about 20 seconds. I shut it down and checked the oil, I was a quart low so I added oil. I started it last night after sitting for a day, no ticking. My question is, since I will have the manifold off should I throw in a set of lifters? The engine has 96K on it, and it's constantly babied and kept indoors. The ticking I've never heard before and the oil is fairly clean. It didn't tick on start up before I stopped driving it so this is something new and so far does not do it all the time. Is there an oil additive that will help or is it time for lifters? I don't want to tear into this again..... Any idea's???? TJ