It's not a big deal and can be done, as has already been said. Yes, You will need 1940 Ford hubs, backing plates, bearings, brake drums, etc. I think your 39 axle can still be used. I think aftermarket repro hubs and bearings are available. Original 1940 drums that don't need turning are getting hard to find. As for wheels the stock 1940 16x4 will work and so will 49-51 Mercury wheels, but they are 15 in. Early Ford 15 in pick up truck wheels work also. If you can get a complete 1940 front suspension it might even have the sway bar. Hope this helps. Oh, one more thing. this changeover will proabably cost you more than anticipated if you find you need new king pins, bushings, etc. Get a copy of the GREEN BIBLE and find all the 1940-41 front suspension part numbers that interchangeable with your 1939. You might even find someone who needs your 39 suspension. Have you checked the Fordbarn classified for parts you need?
All the best.