Two days ago I converted a nuvi 3790 to 3490 as well and managed to enable all features that has the original 3490 firmware What I've done:

1. Before attempting any flash, I've cleared User Data and Non Volume data. Then set the device to connect as a Mass Storage device to the PC.
2. Using the RNG Tool I've made a RGN file to flash with Updater.exe. Using the method with the finger on top-left corner of the screen, I put the device in pre-boot mode and still holding the finger I've flashed the device.
3. Then rebooted the device, it started a little bit slower and without logo. Of course many features like Video player, 3D terrain and 3D buildings, etc. were missing at the first boot. Then cleared Non Volume data again and switched the device off.
4. In order to enable all features of the software, I prepared SD card with the files from the bottom of the first post and insert it in the nuvi. Then switched the device on without holding any finger anywhere on screen and waited the files to be flashed. Then switched the device off and removed the SD card. Rebooted the device, just in case to everything was right.
5. Plugged the device to the PC and started Garmin Express (does the same job as WebUpdater). Downloaded missing voices, themes, dashboards, keyboards, speech recognition data (.srd files in ASR folder, this is the Voice Command), etc. After that I uploaded fresh maps, 3D buildings, POIs, etc.
6. And that's it. Everything is working as it should. I even have made a test in real condition to see how was the accuracy compared to a brand new 3590LMT. The software shows 7-15 meters on the converted 3790, but this is not noticeable in real conditions for me.

To get Speech Recognition Data files via webupdater, you can try to delete ASR folder and check for updates again.