Yes, you can enable JCV in your US 1350. JCV is the only feature not already enabled in it. Firstly upgrade the fw to the current version 6.20 [you can use WebUpdater to make it easy], then the simplest way is to dl the SD card files from the end of Post #1. The thread is long, but everything you need to know and the current links are in post #1 really. Win 7 is no problem, if you have any trouble just run as administrator and/or change compatibility setting to WinXP SP3. If you use the SD method then there is no problem anyway as there is no need to use your PC to load anything to the device.

As for your 255W, you can't use this splash or any equivalent. You maybe would like to convert it to a 465T 'Truckie's Nuvi' and get LA but not JCV [check it's got HWID 0827 and only use the original 2x5W boot.bin]. There are other methods using Asian 2x5W fw, but too many downsides with that imo.