In the shop manual, there is a specific order for putting those bolts in, but even that doesn't always work. Unfortunately, I do not have my shop manual with me at this time, so I can't give you the sequence.
Here is a trick I worked out in the past (and I'm sort of embarrassed that I had to resort to this.)
Insert two flat blade screwdrivers into the exhaust ports. Then wrap a ratchet strap around the engine. The screwdrivers keep the ratchet strap from slipping down the curved back of the engine. Be sure the have a towel or other protection where the ratchet actually sets against the cowling.
Tighten the ratchet until the split is closed and tight. Then use a heat gun to heat the area all up and down the split, out 6 to 8 inches on each side. Run a thin bead of silicone down the split. Let it sit overnight, or as long as you can. If you can let the engine sit out in the sun for a day or two, that is even better.