Flash full 34xx firmware (This means to use the files on the preceding page and flash them using the Pre-Boot procedure by dropping the file in the Updater?
Yes, exactly. Follow the steps in either Post #1 for flashing the 34 fw, or in Post #184 up to 16 inclusive [with my noted amendments to do NV backups as both 37 & 34].

Next, unpack what files to empty SD card
Unpack the file to an empty SD card, turn nuvi off, insert card, turn nuvi on.
remove sd
The one named 'convert37x34.zip'. There are 3 links there in post #1, all are working.

Does something happen between between "turn nuvi on and remove sd? ....
Yep. The command in the file named 'update.txt' will be executed, this one:
xrgn,16,1:/34/16.bin
xrgn,140,1:/34/140.bin
reboot
What that's doing is telling the nuvi to flash the 34x0 logo contained in 16.bin to region 16 and the modified 140.bin to region 140 to enable the additional features. Just make sure you let it finish, it will only take a little while but that last command 'reboot' means you'll know for sure it's done when the unit starts up again by itself. Then you can remove the microSD.

If you don't actually understand all that at present, don't worry, it's perfectly safe [trust me i'm an Australian, we live with maneating sharks and crocodiles, the most venomous snakes in the world, numerous serial killers and shudder.....Kylie Minogue ... we know how to stay safe .

But do as i said earlier and firstly ensure that all features [especially ASR] are checked in 140.bin [except ecoRoute HD & ecoRoute Purchase] using the Mod3xxx.exe app, and also change the v7.20 Ldr.bin to v7.90. If you are still lost, send me a Private Message and i'll help you further without cluttering the thread. But please have a go first yourself, i'm sure you can do it [i'm 66 and badly brain-damaged from alcohol abuse so if i can do it, then you can too].