Lifters can be installed by way of the cam tunnel without pulling the pan using a tool that was designed for that purpose. It’s touchy though as if a lifter drops without the tray in place to catch it, the oil pan comes off anyhow. But it is doable.
.
Here‘s a link to a past thread talking about installing lifters without pulling the engine. A picture of one of the various lifter installation tools that was available back in the Y's heyday is also within that thread.
http://forums.y-blocksforever.com/Topic29290.aspx