Sounds like a fun room update!

Post photos of your great room and adjoining rooms like the kitchen, materials you are planning to use and a rendering of the new room design.

The 18” wide built-ins on either side of the fireplace seem a bit narrow to balance a 50“ wide fireplace. Have you tried a rendering with a 36” wide built-in on one side?

To you have anything from your contractor we could see? As a rule the new linear fireplaces stand alone and are goign without the dated built in traditional look.

With only 18" of room on either side I would not put any cabinets there BUT without a picture I can not say for certain.

Linear fires will always read "modern" and pairing this with cabinet style built in seems quite a bit forced, especially at 18" width. . It's not a pretty proportion, and seems quite unnecessary.

Unless you have a drawing, a visual to convince me otherwise? My advice is edit your design....and leave the 18" empty. Not every nook and cranny of a space merits a "something", and while I understand resources,,,,,,,,not a hack from stock cabinetry, which may be yet more unattractive. Apologies,, but ..no. Save your money and have a better look, a strong focal point.

I need to see the space the TV the FP and the planned cabinets to even begin to help.

wondering how close to the kitchen this installation is? right next to it? Same cabinet style?

Do you NEED storage? What is the purpose of 18" cabinets?

18" shelves above? for what?

How did you diecide on a 50" fireplace? How tall is the wall? What size is the room?

At what height from the floor in the install?

Why 50" when you have more wall available?

Are you putting a t v over it? If so, you definitely want the fireplace to be wider than the t v.