If I leave my van sitting for a couple of days, there is low power and
very load valve clatter for the first 20 minutes of a drive.  If I
start the van up the next day, the power is fine and there is less
valve clatter.

I recently changed 2 lifters and adjusted the valves.

I'm wondering if I have a problem with air-bound lifters.  I'm a
novice mechanic and don't really know what this means.  I've had the
problem for about a month.  I primarily drive the van on the weekends,
so I've encountered the problem about 4 times.

When I put the lifters in, I did not take a scribe and push the bleeder pin in.

The lifters are in the van now.  Does my problem sound like it's
related to the lifters?  If so, what should I do to fix the problem?

I've driven about a thousand miles since I installed the lifters.