Those weld on kits never fit right. You'll just have to use clamps like you said and a big sledge to make it fit right. Don't be afraid to poud the sh!!t out of it. When I did mine, I used a 5lb mini sledge and beat on it as hard as I could swing the thing. It's important that you spend some time fitting it correctly. It won't provide the support it's supposed to if it doesn't fit as flush as possible. Bolt it to the frame first with all 4 bolt holes, then clamp and hammer it flush, tacking it down as you go along.
For fixing the holes, v the cracks as best you can with a grinder or dremel, extend your v alittle further than you can visibly see the crack. Weld it all up and sand/grind it back to shape.