Sweet! Just FYI you'll either need to add a block of wood so the thru-bolt provision lines up with a surface of get rid of the block of wood currently holding the arm. You'll also need a thru-bolt since mast arms don't come with hardware as far as a know (except for those NEMA head kits that include everything) Any long bolt with a washer and a nut that you buy at a hardware store will work as long as the bolt is around the same demensions as a true utility company thru-bolt.
I'm not positive, but I don't think so. A 100W MH ballast with the ignitor disabled drives a 125W MV lamp nearly perfectly. A 100w MV lamp would be slightly overdriven (it will work but a 70W PSMH ballast is required to run a 100W MV lamp perfectly) and a 175W lamp would be underdriven. Being underdriven, I believe it'll either not warm up all the way or make the ballast run hot.