Been there, done that. Real diamonds cheap quite easily regardless of what the movies say about it.

My personal opinion in the matter is not to cut or alter the glass baffles in an operational sump/aquarium. Why risk the following.

a. injury/cutts/infection etc.
b. increase possibility of damaging the side panels of the sump (while its still holding water)
c. small shards of glass left in the sump afterwards.

I'd purchase a new tank/sump, make the conversions your intending to have and then replace it with the existing one. It's a little more effort and cost, but why take the chance.

update
 

Ended up using a hammer to break the glass. Worked fine and everythink is ok.