I'm assuming you mean a 29 INCH cat step hull with the rest of the specs you've included. Now, as far as your questions go:

• 6S will probably be fine with a 1800KV motor. What kind of batteries are they?
• Not sure where you're getting the 10,000MVH from since voltage isn't measured in hours. Could you be referring to mAH, which is a measurement of amps over time?
• I don't see why a 150 amp ESC wouldn't work in your configuration, depending on the battery type. 120C puts out a lot of power
• A 40mm prop at what pitch? Higher pitches require more power to push but will give more speed than a lower pitched prop.

Somehow I doubt you will reach 70MPH since a club in my area(Seattle) races 1:10th scale hydroplanes that are slightly larger than your cat and they don't get anywhere near 70, running 4S packs