I don't recall having to trim those. Here is a shot of the layout of rivets on mine. Word of caution here. Make sure the holes on the gusset don't line up with the end of the underlying steel brackets. Make sure you have the edge distances OK for each hole you mark before you drill them.
Yes, they were pre-bent, but the forward (firewall) edge will need to be trimmed to fit flush with the firewall. This angle is established by the firewall bracket, so I know it's correct.
Tom
You'll have to trim the front of the gussets. Like Bruce said above, pay particular attention to the edge clearance on the steel below the longeron. I had to make sure they were tucked in really tight inside the angle. If I recall correctly, I radiused the steel edges slightly to match the contour of the inside radius of the longeron.
Thanks everyone for your input. I trimmed the forward end of the gusset and made it fit nicely. Thanks for the warning on the edge distance that was a big help.