66 inch tub in a 68 inch alcove?
Build a filler shelf or projection. Nicely done it can be storage for bath gel etc

Or you can use it for candles for added ambience...

And had to do a shelf like that for an 8" gap due to the size of the bathroom and the cast iron bath that needed to be retro-fitted. Some nice stained wood and tight edges with sealer and it is not a problem. Also had a tiny slope from back to the front edge so that any splashed water does not sit at the back edge.