G'day Vince,
No way to edit NT map format to remove anything like that sorry. But don't blame Sensis for this map, this map contains HERE [Navteq] data. [Only registered and activated users can see links. ] is similar size, ~257.6MiB [~270.1MB].

But there is a way to up the Garmin data card memory size to 2GiB by replacing the NAND chip in it. Read this post for more info: [Only registered and activated users can see links. ] [post #25]. It's seemingly a 4GB chip partitioned to 2GiB, but your unit can't use a map or chart img file over 2GiB anyway afaik.

One of our members with a GPSMAP278, Doc_Kooindinate, recently purchased one of the upgrade chips for RUB6000, presently around AU$140: [Only registered and activated users can see links. ] [post #9].