Contactor/relay rating for bypass function of soft start circuit
The contactor will carry the full load current when pulled in, so should be rated for at least 70A, maybe 110A as that is what your circuit protection is rated for.

You would probably also want a second contactor on the output side of the transformer to prevent load being applied until after the soft start has completed if doing the resistor thing, as otherwise you will likely blow out the resistors, the NTCs have an advantage here. 

If you do the resistor thing, make sure the resistors are rated for sufficient pulse power handling (And voltage), you may find specialist carbon composition parts helpful here.