Door Jamb Solutions – Wider Than Framed Entry Issues

From you description, it sounds like you have a hair over 1 1/2 inches of extra space to make up. If this is correct consider this method. Place a full 1x4 (or whatever the depth of the wall may be) on both sides, as long as the new door frame will still fit comfortably in the new opening. When ever possible, I like to mount the hinge side to a solid petition without excessive shims. This makes it more solid without having to bridge a large gap with your finish nails, and makes it a lot easier to perfectly plumb the hinge jam. I often replace the center hinge screw on the jam side with a longer matching screw that will attach to the jam. This does a few things for you. One, you don't need as many nails, gives a much more solid feeling and sounding door, and gives you a tool to fine adjust the plumb if necessary.(especially on heavier solid wood doors. Just like with security screws on an entry door. After you are sure you are happy with the plumb on the hinge side, simply use visual to get an even gap on the top and latch side, in that order. You can then use shims on the latch side and shoot your finish nail just below your shims. this keeps the shims from dropping as you work down the jam. i don't usually shoot through the shims because they often split and I may want to slightly adjust them before the trim casings are installed.

Your plan to fix the gap in the center of the door sounds fine. I might suggest using some shingle shims instead of flat stock however. If you want to just remove the interior trim only, that is fine also. Using the shingle shims, place the thickest end in first and by trial and error, adjust the opposing shim to a size that will snug in nicely and then by gently tapping it in next to the first shim, remove the excess gap. Put a small finish nail through the jamb and shims to prevent them from moving. Drilling a pilot hole will assure not splitting the shims with the nail in inserted. Now just trim off the excess and reinstall your trim. Job done! Good Luck