You might think about ordering it with the 6", and remove and take to a sheet metal shop to have cut down off the bottom. Pretty sure they just attach with 5 or 6 screws, so new screw holes would need to be drilled as well. Or, if it's only a half inch, perhaps the legs on the range can be lowered to the minimum to get the clearance you need?

I don't think you could remove material from the bottom of the BG as that's where the mounting tabs/locations are. Probably better to remove some material from the top and have it reshaped. I just installed a 21" on mine, the burners need to be removed to get to the mounting flange which has the screw holes in it, it's a 2 piece, hollow unit (oven vents thru it) and mounts to both the back of the range and behind/under the burner grates. I think it would be relatively easy to cut it down from the top but like I said the oven vents through the back guard so I would want to know what's limiting it to under 6" in your case so that it wouldn't be damaged by the hot gases coming out of the BG.

Or you could just use the smaller island trim blackguard, which is what I have in my bluestar and just bring the tile or whatever backsplash you have down further.

We have a soapstone window apron and sill above so had wanted to tuck the stove in just under the stone. Need it slightly shorter.

...or order with the island trim, and have an extension made for that?