No crossmem below the mounts. That would probably help too, if it can be fit in. I would most likely add 3/8-1/2" plate along the side of the frame, and probably the top/bottom too. Either inside or out, wouldn't matter which.
The vertical crack I had was due to the weight of the engine. More than the frame can handle, suspended midpoint between the spring ends. Every bump on the road pounds the weight of the engine down on the frame, flexing and weakening the frame. Somewhere I have a pic, couldn't find it this morning.
Plastic trucks suck...
Iron trucks rule...