If it's only flashing 3400 it's not a 3600 converter unless it's an off the shelf converter. Still should be loose enough to let it idle good. My 360 has a cam just a little smaller and it idles super with just a 3k converter. Drops from about 825 in park to 700 in gear.
I wonder if the springs in the dist. are too loose letting the mechanical come in at idle in park then when you drop it in gear the rpm lowers just enough to drop some timing out? Get somebody over to help you and put a timing light on it and have them drop it in gear and watch what the timing does.
