I don't know how much cutting you've done, so please ignore this if you know it already.
A possible strategy: First thing is to make a pair of replacement guides to match the ones you are about to snip, sometimes a little insurance is all you need. After removing the guides, mark out the part you need to cut out if there's nothing marked to guide you. There should be a corresponding shape on the resin to help you get it right. If in doubt, make little paper templates the exact shape and stick them down to draw the guidlines. Remember to stay inside the lines to leave the actual cutout a tiny bit too small, so you can snug the resin part in after a bit of adjusting, sanding sticks work great for the final adjusting.
Those two corners at the front, you might want to drill them right through first, remember to keep the drill radius on the inside of your corner guide lines. That makes for an easy start and finish to your cut. When you cut, start at the hole and go down just past half way, don't go all the way to the other drill hole. The same the other way 'round. That way you don't accidentally cut right through the fuselage's wedge.
Don't try to cut all the way through the first time, if you are using a new xacto blade try and resist the urge to cut too deep. You might need up to 12 scratching cuts to get through. If you are using a bit of saw blade, you will definitely want to start with those drill holes, and remember to stay inside your lines.
Second cuts should be from the holes going backwards to the guide side, not the other way around, again, you don't want to accidentally knife over the wedge details. Last cut should be easy, like the first cut, start at the cut and don't go much past half way down to avoid over-cutting.
Hey if decide to bail, you can glue the 'insurance' guides back on and save it for another build, and use very thin sheet styrene to cover the cutouts.
If things go really, really bad Rich, I'll have my bro send you some M-80's