These studs don’t need to be especially high tensile – they only have to hold the manifold on tight enough to stop it leaking.
My 2 litre has UNF threads here & there – head nuts for one. Some are marked UNF (not the head nuts I don’t think).
And of course there are UNC threads, which are almost all the same pitch as Whitworth, (one size is different – 11/16″ maybe?) but with a slightly different thread angle or profile.
You are assuming that you have 5/16 Whit studs (not 1/4!), but they could be 5/16 UNC for all you know. Frankly, after all they’ve been through I doubt it would make much difference which nuts you use!
I’m currently in America, where I have a few old cars, including a 1961 Corvette I’m currently rebuilding. GM make their cars out of heavy metal stampings & bent bits of metal held together with UNC bolts. It was such a pleasnt contrast to work on a properly engineered car like the Lagonda. Although I have to say the Chevy is a lot easier & quicker!