Well I cannot explain it but I have seen similar engines run completely differently , for example my 440 Duster would barely warm up & would sit at 180 * all the time while my friends 440 Duster making less power was always in the 200- 210 range
bigger rads & more efficient fans obviously help adding air flow & coolant capacity , , my friend locally has a 340 Dart & it is the same as yours , always above 200 idling but it drops as soon as you move but his rad is on the smaller side for sure .
generally removing the T stat hurts cooling as the water mover to fast through the rad & doesn`t cool as much as it should , the idea is to swap the water out , letit sit in the rad for a minute or two with the stat closed then swap the warm water with the cool water , not just cycle through constantly
I am not sure what else to recommend you try other than a larger capacity rad