I have a m47 ballast with multi tap capability and wanted to use it to power my 480v 700w OV-50 with a 120v input. Has anyone tried this because it seems like it should work but I'm not confident electronically.

I dont recomend it.  It would work but it not recomended because it not isolated.  And event somthing goes wrong case. 

 I would recomend that u get proper transformer 120  to 480 V
At least 2 KVA.   I found mine at Restore and paid 5 buck for it
These can be sold for 2 or  300 bucks on Ebay.

  U can how ever reballast your street light and reduce cap value by
 30 percent to run 700 watt MV.  And none of this step up conversion.   

 Only reason I wanted a step up is two things having a real 240 volt to neutral like rest of country and also provide 480
Volt for my street lights.
   

I've done it at a much smaller scale to run 240v incandescent lamps. While not Ideal, an m47 ballast should work as its 1000w. It's best to use a ballast larger in wattage than the thing you want to power so as to not overload it. Just make sure to cap off every other voltage and the secondary and you should be good. The best thing is still a dedicated transformer capable of powering them up, but this can work in a pinch for quick testing.

Yeah not ideal but should work ok in a pinch, just make sure the ballast is higher wattage than the ballast you're trying to power, you might want to correct the power factor on the 480v ballast with a capacitor to lower the VA drawn. That will lower the strain on the ballast you're using to power the light.

 Maybe verify you have the correct voltage with a multimeter if you got one. Just make sure you're on the correct voltage setting and your meter is rated for the higher voltage.

Also a fuse on the primary might not hurt if you got one.

Definitely get a fuse fitted, otherwise you might be the one getting hurt if things start going wrong...

I never use a ballast as a transformer. Instead, I use two identical ballasts in parallel to drive a light source with twice the power of a single ballast.