Another option: I installed '70 Mustang seats which are high backs, in my '67. That way they still look original to the era, but with some added safety. Your wife is quite right to question the matter.
An upholstery kit supplier in the USA was quite happy to supply me a set that was a mix of '70 fronts & '67 rear, in the parchment colour I wanted.
But if you'd prefer more modern seats, then perhaps look at using the Mustang 'Fox-body' type, ie. '80's era; see the article in a recent Mustang Monthly mag as noted in a previous post about installing them.
Maybe your car supplier in the USA could chuck in a pair of '70 or later high-backs for you, shouldn't cost much if they're rough.
Note: If you do use more modern seats, like from an AU ute or something, then you need to watch the headroom, & also clearance for your knees under the steering wheel; so try for fit before you buy.