Henry's 70 Dart Swinger

Most of the time, if you're idle is dropping that much just by putting it in gear, it's because the vacuum advance is adding timing at idle, then when you pull it in gear, the vacuum signal is reduced, you lose timing, idle drops...double hit.

If the engine is tuned decently with a small cam, you should be able to idle it down easily to 700-800 rpm and only observe a minimal drop in going into gear-maybe 100 rpm.