The 8 may be different, but on my 7 I don't believe that slot was originally in the design. The idea is that in a crash, if the wing is bent back the bolt will pull out of the slot before it ruptures the tank.
I guess if I didn't have it, I'd wonder if the juice was worth the squeeze to install it after the fact if I didn't have some other reason to have the tank out.
As far as cutting it it in place, if it's aluminum, so you can use an abrasive wheel on a dremmel or die grinder without making sparks. As you point out, you could also just drill some holes and play connect the dots with a handheld hacksaw blade etc.
Whatever you decide, you need to dress the edges smooth afterward to eliminate potential places for stress cracks to start.
On the RV7 that slot is in the bracket on the fuselage side, not the tank side. But in that case, the fuselage bracket is steel, so that might be the difference. Might be worth a call to vans to see if it's reasonable to cut it into the fuse side bracket on your plane. That would make it a lot easier.