Figure 1. Parts obscuring the head nuts, these must be removed

Figure 2. This is what you need to get to! (Click the picture to enlarge.)

The following is a procedure that has been used successfully in the past.
Step-by-step procedure:

1. Remove the spark plug caps.
2. Remove the spark plug(s), which may have a CHT sensor ring and lead under them.
3. Remove the two rubber mounted bolts that hold the stainless steel muffler bracket to the engine.
4. Loosen the two long bolts and nuts that tighten muffler bracket around the muffler and carefully rotate the muffler bracket down, around the muffler, and away from the engine to gain just enough clearance for a torque wrench. This is a good time to check the health of these bolts as they tend to wear their shanks due to vibration.
5. Remove the two remaining bolts that are holding the shroud to the heads.
6. Loosen the sheet metal screws around the perimeter of the shroud and then pull the shroud downward enough to clear the screw heads and then rearward away from the engine. This exposes and gives access to the head nuts for the retorqueing.
7. Using the ‘star pattern’ described in Figure #2, loosen and retorque one head nut at a time. Retorque to the full 195 inch pounds.
8. Replace the spark plug(s) and torque to 240 +/- 22 in/lbs.
9. Reattach the shroud putting a dab of blue Loctite on each shroud screw. This helps to keep them from working loose and possibly going through the prop in the future.
10. Replace and moderately tighten the two rubber mounted bolts that hold the stainless steel muffler bracket on the engine. Tighten the two muffler to bracket bolts. By using a length of square or round tubing here to form a spacer through which these bolts pass will provide something for the nuts to tighten down against without deforming the bracket and quite possibly result in less vibration damage to bolts. Use a little blue locktite on the bolt threads.
11. Lock-wire the two muffler bracket mounting bolts together to prevent them from backing out and possibly flying through the prop at some future time.