Sluggish 904 when cold or converter?

It's actually pretty normal. My '74 does it.

The converter drains back, and since the pump doesn't run in Park, it doesn't start filling back up until the trans is in gear or neutral.

The next time you have the chance, start the engine in Neutral, let it run 20 or 30 seconds then shift to reverse. I bet it engages firmly and takes right off.