Depends on how thick your acrylic is. You can you would have to empty out your tank and use a router to remove it. 250 isnt as bad as 400. Is it HQI or Mogul? IME Mogul bulbs gets alot hotter than HQI.
I had that light (if it's still the one I traded you) about 4" off my acrylic and never had a problem. add a small fan to keep the heat from the bulb moving in between the bulb and the brace so it won't build up and you will have no issues. It didn't melt or craze mine and it was 3/8 acrylic.
Can’t you just glue in a new brace, maybe one on each side of the area the light is now and then cut out the middle brace with a rotozip or router? Just a thought I never tried it but it looks good on paper. I know you have to use the special acrylic glue but then you would not have to empty the tank.
You would need a clamp big enough to squeeze the tank back together. Or weld-on them in with the existing brace and then cut it. However that doesn't fix the problem of having bracing in the way of lighting.
I don’t think you would need a big clamp if your already welded the new braces into place before you cut the one in the center. The center brace would act as the clamp I have black braces in the top of my tank but it is not a problem with the light because they are not directly under the MH lighting. I don’t see any shadows and I doubt you will if it is possible for you to brace your tank off to each side of the MH lighting.