zekenator,
There are four holes available for bolts or screws so the bottom tabs are not mandatory. That is, a little careful Dremel work would eliminate them. But first, of course, you should test fit the custom frame a time or three to confirm if it could fit without modifying the factory mounting bracket. If the tabs have to go then they have to go. To restore things to factory condition, a new OEM part or one from a scrap yard would work if it is in good shape.
Can you please post up a photo or two so we all can see what you're working with? Somebody here will probably have a better approach than removing the tabs. You can't be the only one who has put fancy plate holders on their ST.
HTH.