Sic, I'm no expert but can I suggest that sometimes you're better off cutting more out rather than the bare minimum. Also instead of cutting random bits out pick a defined line, straight or curve and stick to it. Cutting more out will allow you to more easily create the inner arch, working from the outside rather than the inside and cutting the curve of the arch out will make it easier than random in and outy bits.
Is that wheel arch repair piece one of these universal jobbies?
I have 4 mini fibreglass extensions.
You may need smaller wheels though.
Edit, they are massive, must be for 13" wheels I think.
Mini arch 001.JPG
Just measured and the width is good but they're not high/wide enough.
You may need to get some wide 13" diameter slotmags tho.
Actually doesn't look that difficult to do, make templates, transfer to sheet steel for outer arches and 'closing' panels, and Herbert Austin's your uncle. You don't have to worry too much about originality, period 70s custom style and methods. Bit of fun and enjoyment making it all on a budget and learning as you go along.
Just an idea. Could look quite neat on a 2-door ADO16. Pics 2 and 3 tell the story.
SiC I'm really impressed with your progress with this car given that you'd no previous experience in this type of work.
The nearside has been previously repaired in a similar fashion.
I am quite fancying doing something with those arches and customise it. Not quite to Vulgalour levels but a mild bit of fettling.