If it was mine, I might fix it myself just to see how long it would hold up. I've had great luck welding cast iron with a Tig and SS filler rod.
Stop-drill all the cracks, and v-groove and weld. 99% of that manifold isn't structural (I wouldn't lift a motor with it), and the most pressure it sees is 16psi or whatever your radiator cap relief is set at. Thermal contraction and expansion are your biggest enemies but since it isn't structural and it's a huge heat sink, it might be fine. Worst case, you get an intake leak or a coolant leak. Intake gaskets fail on the regular and nobody dies from it.
Try it and see. I just wouldn't pay some other slob to fix it.
