Good evening Gary,... Let's start with the simple solution. Look on the inside of cabinet doors and see if there might be a sticker indicating what company manufactured the cabinets. If your lucky and can find that information you can go to one of the "big box" home improvement stores and order two toe kick spacers to match your cabinets. They will have them in 3" increments. If you do have exactly 6 inches to fill (3 on each side) you will need to trim each one 1/8th to 3/16ths to allow room to slide the range in.

Now if these are hand made cabinets or you can't come up with the manufacturer, your involved with a much bigger task. I can give you a step by step to build a tow kick but before taking that time let me know if you do find the Mfg. CO.
Also when you get back to me give my a description of your cabinets and if possible what type of wood.

        Thank you for the question, Dennis

Good morning Gary,... Okay, so were out of luck on a manufactured one... First I'd like you to slide out your stove, enough that you can view the side of the adjacent cabinet. Purpose is for you to get a good visual impression of the profile of the front end of a base cabinet. You'll see the front edge coming down to about 4 inches above the floor and then going back toward the wall at a 90 degree angle (probably 3 inches) and then down to the floor at another 90 degree angle. You will be duplicating this with approximately a 2 7/8ths inch wide finish board. I'm guessing at this width. You will need to verify exactly what your stove width will need to be to be able to slide it in once these new toe kicks are in place.
Select, from you local home improvement store, a 1 x 4 piece of #1 select pine. You could choose maple if you prefer. This finish board will probably be 8 ft in length. Depending on the height of your custom cabinets you will be using about 37 inches for each toe kick.
Cut your finish board into two pieces of 38-39 inches each. Now rip these to the exact width we've previously discussed. The next cut off would be the cabinet height from the top of the cabinet to the bottom where it makes the first 90 degree turn toward the back wall. Should be right at 30 inches but please verify. The next cut off will be the return piece which will most likely be 3" minus the thickness of your wood. Next cut will be final distance down to the floor. Probably close to 4 inches plus the thickness of your wood.
Join the shorter return piece to the front piece by pre drilling two holes from the front of your long piece into the short piece. You shuld counter sink these holes in the front. Apply a good quality adhesive (like PL 400 or liquid nails) to the end of the short piece and screw in the screws. Same process for the back piece that goes to the floor but counter sinking won't be necessary since this attachment is out of sight. Next step is to pre drill and countersink three holes along the edge of the longest piece and into the rail of the cabinet edge it is joining. (top, middle and near the bottom) When this gets installed you will also apply adhesive to the attaching edge. Now all you have to do, is do it all over and you've got your two new toe kicks. At this point I'd recommend applying primer and the first finish coat of paint to the exposed surfaces. (fill the counter sink holes at this point also)
Once these are installed to each side of your stove opening you won't need to run a brace from the front to the back. This stability can be accomplished by the attachment of the new countertop. All that is left is the final coats of matching paint. I suppose the front surface of your existing toe kick is black but if not just match this also.

This should get you where you want to be so please select the "accept" button but still get back to me if you have additional questions.

        Thank you much, Dennis (cut2it)