From the few years I've lurked on this board, it's common for a high mileage 300 to make a little lifter noise - mine has since I bought it 3.5 years ago. When do the lifters make noise? A little ice cubes in a glass sound at start up is ok as long as it doesn't persist - meaning take it for a test drive. If the noise continues after a few miles (10 or so) and you notice it persists at idle, then be concerned. I've put on over 50K mi on my old '79 stocker, have crept past 220K and have had no other problems other than a little lift noise at start up.
I was once told by a machine shop the noise was due to increased valve train tension, being that the valves begin to seat deeper in their seats....replacing the lifters won't fix it. I personally think it's due to low oil pressure at start up. A high mileage engine always benfits from a good warming before driving.
If you really think swapping lifters is the way to go be warned. A high mileage engine with the original cam may not have enough pattern to break in a new set of lifters - they won't spin in their bores properly or at all.
The 300 is a tough, dependable motor. Don't fret on a little momentary engine noise.