Bottom line Roger, is if you don’t fully understand the engineering, and are not prepared to risk your chances with vendors and back yard mechanics who think they know better, stay with what the extremely competent and fully tested in service solutions that Jaguars engineers decided on when they were making our XKs, and taking responsibility for their design and manufacturing. Modern engineering is rarely better overall, as goals these days are for cheaper solutions, with no conceen re longevity in life of mechanical components. For con rod bolts, best to stay with original old bolts, but realistically checking condition for damage/corrosion if bolts are sourced loose. If you are pulling down a sound but worn engine with no bottom end failures the bolts will still be within original design parameters. Same comment really slotted nuts, but of course you must use correct and new split pins.
A problem in Australia is modern new split pins are invariably metric, or typical Chinese marketing also called an imperial ‘near enough’. They are not. You MUST acquire correct diameter imperial size split pins, and then know correct way to install them. Head sits inside slot, and legs fold back over end of bolt, and forward against side of hex nut.
A little bit of commonsense and care, and you will get a perfectly adequate and long life result. If you choose to modify, or listen to the vendors and the back yard mechanics trying g to sell you non original bolts and nuts, ask them to show you why they are better engineers than Jaguar employed in the 1950s.