You can usally buy solid mounts for ford mustangs and fox body cars. Rubber mounts let your engine get leverage and then snap the mounts. Also check the trans mount if its wasted it can put pressure on the other mounts. A solid trans mount or one from energy suspension made out of neopreme will help keep things in line.
Lots of hp may require a turn buckle or even a chain to hold it in place. Turnbuckle kits are easy to hide and will hold the engine agaisnt the frame. Front mount engine plate mount will also resolve the issue.