Would high-quality 1" spacers on each side allow you to use a standard wheel of your choice for that year or are going custom anyway?  I would prefer not to use spacers if possible myself in the KISS approach but they can solve problems and can't say that I won't be using them when I find wheels for my car.

I did a few searches - what else is there to do on a Saturday night!  Not sure but perhaps Dearborn Classics, which are claimed to be made in the US and welded may be able to make you two with 3.5" backspacing.  Vintiques through Summit have 4.5" backspacing in the sizes you would be looking for.  This is out of my range of experience but it looks like you are on the right track as far as offset - I think you may have a hard time finding them at a reasonable cost if at all.  There is one narrow Vintigue 54 series with 3.75" but it is 14X6.  The Dearborn Classics are about $100 more per wheel and make claims regarding their quality over lesser brands.  Others may have knowledge of original wheels that may work.