Marble / Slate fire surround question

[Rake722]

Hi all, I have found a lovely marble fire surround that has been removed already from a Period property and I would like to install it in mine. The question is is can the width of the openings on this type of fireplace be adjusted? I need at least 900 mm between the operates however in its existing configuration it only has 860 mm width of the opening and I noticed on some website that sell these things they mention that on some fire surround the width of the opening is adjustable within reason.

I have included a picture of the very Fireplace in question

 

[Cubist]

If you're lucky you may find that the section you've highlighted with the arrow is in effect 'tennoned' into mortices in the columns AND the tenons are long enough AND any new material that would then be exposed matches that of the rest of the surround then the only remaining hurdle would be the maximum width of the steel/iron fireplace insert. Theres no real way to be certain until it all in bits I'm afraid and the current owner may not be willing to do that for you.