That all sounds right. I would try using Mapsource instead of Basecamp/MapInstall. That is the software originally provided for the GPSMap76, Basecamp was released later. You can download here:
http://www8.garmin.com/support/download_details.jsp?id=209Mapsource should see the same maps that you have already installed in Basecamp. For starters, just send one map and see if that works. If so, then you can send them all. Here is the Mapsource tutorial:
https://www.gpsfiledepot.com/tutorials/how-to-load-maps-on-my-garmin-gps-unit/#mapsourceNow there is one other possibility for what could be happening. These older GPS units have a rather restrictive "segment limit". Segments are the smaller tiles that make up the entire map. Your GPS can only access about 1000 segments. There is no standard size for a segment, it's up to the mapmaker.
In one of your posts you said the file was 600MB, but in the other post you indicated it was 600KB. A 600MB file might contain more segments than your GPS can handle. When you exceed the limit, unpredictable things can happen. However, I would expect that at least some of the map would work.
Anyway, Mapsource will tell you how many segments are in the map you're sending to the GPS (in the left pane of the window). I'm not sure if the current version of Basecamp/MapInstall shows this info.