I'm fairly certain the D3599xxxA.JCV files are for 30/4x/5x series nuvis and StreetPilot phone apps only. Those files have a different encryption/file construction to the 'standard' jcv files used by other US/EU fw units and therefore cannot be used in 765.

Nuvi 765 can handle jcv with picture size of 212*234 and larger. As the D3568xxxA.JCV files are the split-screen type i guess it will be 'stretched' to fit the screen and may appear a little washed out because of the lower resolution exactly as it will do in the 1xxx series which also can only display in full screen. Any full screen 480*270 will work in both 1xxx and 765 interchangeably.