The best way to get a 390-ish feel with the Targas is to remove the C-Flex inserts from the ankle strap. It'll be a softer flex, more similar to the 390 ankle strap.
It won't feel exactly the same, but that's as close as you can get without picking up a pair of 390 Bosses. Both bindings come with the ankle straps in the center position out of the box. You could move them to the forward position but it won't affect the flex very much, it's mostly just for sizing.