No issues with inner fenders, I actually removed fenders and inner fenders.
I also had to straighten the driver front frame rail, it had minor damage in the first few inches.
Yes, take multiple measurements multiple times, then measure again, test fit, repeat.
Keep in mind not all old cars are perfectly square and even, either from the get go or after 50 years.
I have no recollection of where the parts came from, but that pic looks like them. They were quality pieces.