I have worked with old timers that are hacks, man. "I've always done it this way" is NO excuse for doing **** work. Some guys fumble their way through life not by their great skills but by sheer luck and opportunity. Being old and experienced does not mean that the man is smart and competent.
Hedman Hedders are not the cream of the crop. They are not the worst but the cheaper price usually means that dents and dings are needed to fit. Doug's and TTI headers are more expensive but they fit. You are not likely to have problems like these with those brands.
A mini starter is smaller in diameter AND shorter but that is only part of the problem. If the solenoid is clocked wrong, you may have LESS clearance when using the wrong one. Some position the solenoid toward the top, close to the engine block. Some place them toward the drivers wheel a bit more. Ordering one online can be tricky. I had similar problems years ago with starter/header interference. I called Mancini Racing and asked them to pull a starter from stock and describe the position of the solenoid. When it arrived, even it did not fit. The safest way might be to physically see the starter in front of you.
The 400 engine is a LOW deck big block. The exhaust ports sit about 3/4" lower (Closer to the starter) than a 413, 426 and 440. This may have a negative effect on header clearance