My suggestion would be to set it up as:
1. GROUP (transmission, engine, rear axle, etc)
2. VEHICLE COVERAGE (what make, model year, type, etc)
3. ISSUE DATE (this needs to be down the list as some bullitens were issued after the end of the particular model year, like an item covering 1949 models issued in 1952)
An option would be to create an index hyperlinked to the individual bulletin files that lists vehicles covered, subject and bulletin #.