Like you, I'm using Smiths gauges (mechanical, except for the tach, which is an RVC-type MGB item), and getting them to talk to other, more modern systems is, well, challenging.
One place you might try, if you haven't looked there, is Chris Spiyda's site
https://www.spiyda.com/magento/index.php He has designed & built all kinds of electronics wizardry to connect up various British bits to modern bits. I've just purchased one of his tachometer-matching black boxes which, it appears, will allow virtually any electronic tach ever made to be accurately driven from virtually any electric or electronic tach source ever made. He also makes black boxes to make various fuel senders work with a plethora of different gauges.
Worth a look, and the reviews of his products are outstanding. He's also easy to talk to, and happy to help sort out your problems (mine was how to get a 1970's RVC MGB tachometer to talk to a bespoke, modified GM OBD-1 ignition source, while fitted to a Ford Zetec engine!!). He's supremely confident that his solution will work.
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Scratch building, at continental-drift speed, a custom McSoreley-design framed, dual-Weber 45DCOE carburated, Zetec-engined, ridiculously fast money pit.
http://zetec7.webs.com/