Before tightening top mounts, check to make sure there is no space left between the mount and rocker box (or heads respectively).
Don't allow any of the mounts to be “stressed” or “pulled” into position by tightening the bolts.
The mount is flat at each bolt hole and any space between it and the recessed hole on the rocker box can stress therefore break the mount.
If it doesn't break the mount, the stress can allow the rocker box to be “stress” pulled upward and create a leak at the gasket.
1)
Use extra washers to add / change as needed to achieve “zero space left” before tightening. Remove the mount if need be to add shims.
Then go back through the installation steps to just snug the entire assembly before torque is applied.