#1
I have an Ibanez GRG120BDX that i picked up for around 90$ to modify. I changed out all cosmetic things (knobs, pickup rings, jack cover, and truss rod cover), electronic things (new BKP pickups, Jensen Angela Cap, and BKP tone and volume knob), and other hardware upgrades (new nut, new springs, and new tuners). I've spent a lot of money on this guitar, but i never changed the neck because I liked the inlays and the feel of it. However, I would like to change the bridge out for something much better. The problem is that I have no clue what will work. It is an Ibanez Fat-6 so it has a weird screw placement (3 on one side 3 on the other with a space in between  - - -   - - -), and it also is a vintage style tremolo. I need help figuring out what bridge would even fit in its place without needing a major adjustment. I know nothing will probably fit perfectly (IF THERE IS PLEASE TELL ME), but I want something that would at least keep the outer screws in the same holes. It would also be appreciated if you could tell me how to drill new holes without the others causing problems.     
METAL
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#2
You can fill the old holes with match sticks and would glue. Sanded flush obviously. As for what bridge I have no idea.
#3
It would be easy to plug and redrill the holes, but the bridge doesn't look like a standard Fender size to me. If you can get hold of an old fender style bridge you could check the whether it would fit in the Ib trem block cavity and whether the saddles could be correctly located. You could then order a decent one with reasonable confidence that it would fit OK.

Why do you want to change the bridge? I just content myself with new pressed steel saddles.
#5
do you want a trem bridge?

if it were me I'd probably just plug the holes and put a hardtail bridge on, similar to the one on an RG321.