OK, I think I now understand what you were wanting. There's no reason to use an old firmware version to do the re-conversion 34xx > 37xx. Use the latest V5.10 GCD from the perry link above, or get it direct from Garmin:
Convert the GCD to an RGN with RGN_Tool as detailed in the first post and then flash it in preboot mode using Updater.exe. You'll need to follow that tut in initial post, but as a guide. Re-load 37xx logo and files. Post again if unsure. May I ask why you want to convert it back, are you having satellite accuracy issues?Code:http://download.garmin.com/software/nuvi37xx_GCDFile__510.gcd