Width shouldn't be an issue, bolt hole alignment might be, but not a deal killer. There were two gauge cluster styles for '40, the Standard being a simpler design, all the gauges behind a single pane of glass (the most common style one that Classic Inst used for inspiration if you're meaning the one with the speedo and tach in the center portion). The Deluxe had a plastic cluster face with individual squareish openings for each gauge.

On stock dashes the '38/9 were effectively a flat panel all the way across with the two large round gauges as you mentioned. The '40 was a much more "complicated" style, being rounded top to bottom, protruding toward the front seat riders with more texture in it's design.