Ideally, your chainline would be 45mm, but 48mm is what you have to work with. You could change the spindle to move to 47mm or 46mm, and that may or may not and may or may not shift. Your bicycle was not designed to accept a triple, which is why you are having problems.

Is your front derailleur "period correct" for a triple crank, or are you trying to use a double derailleur? The chainline can be slightly off without any real problems, but you just don't have an appropriate derailleur. Did a 1974 Raleigh originally come with 126mm dropout spacing or 120? If it is 120mm, then the front derailleur it came with was only built for a 42mm chainline, not the 43.5 of 126/130 double crank spacing or the 145 of modern triple.

I think you are going to have to live with a slightly off chainline and find a more appropriate derailleur. The only real alternative is putting clearance dent in your chainstay for the 30t.