Who makes the best detailed single 5"/25 cal open anti aircraft gun mounts in 1/350 scale? I am wanting to replace the ones in the Trumpeter USS Lexington.

Give the 3d printing a shot. they can print in any scale. Contact the and see if designer if it's plausible..

https://www.shapeways.com/product/4PBV4KK2D/1-200-5-inch-25-12-7-cm-deck-gun?optionId=61082441

I must warn you the resin it brittle. Do any finishing work to do while it's still on the tree/sprue. Then cut the sprue away from the part(don't try to cut it real close) and work your way cleaning off the sprue towards the piece.