Exo,
I am not familiar with the Wells Gardner D9800, so not positive, but I think I can help you out. I have a Mortal Kombat II cabinet that I am MAMEing, which I think is the same Konami cabinet that TMNT came in. I got the cabinet empty and was planning on building a shelf and putting in a TV. Well I was able to get a Wells Gardner D9200 monitor and of course decided to go that way.
I had the same issue the cabinet has mounts for a 25" monitor and the D9200 is a 27". I was able to pretty easily modify the monitor mounts in order to be able to use them with the D9200. You can see the steps I needed to take in this Project Announcements post:
http://forum.arcadecontrols.com/index.php?topic=87749.0Basically I created a template for the monitor, shaved off the posts on the mounts. Used the template to drill holes to match the monitor mounts and then had to trim the mounts a little to allow for the larger monitor. I am not sure if the D9800 has the same mounting holes as the D9200, but I would think they would be very similar if not exactly the same.
The part that you trim off is small and does not affect the way they attach to the cabinet and the monitor is very sturdy and secure. Let me know if you have any questions.