The OEM 1-1/2 pass configuration will work fine when kept in tip-top working order. The primary reason to convert to a single pass configuration lies in what happens when it’s not in tip-top working order. With the OEM 1-1/2 pass setup, the upper 1/3 of the radiator can get plugged up and cause valve seats to start rattling in the B bank while the A bank is running fine and the temp gauge is indicating all is well. With a single-pass setup, it takes more crud to stop up the radiator to the point of trouble (if you opt for an aluminum radiator it’s all but impossible), and when it gets stopped up the symptoms are as you’d expect, creeping higher temps on the gauge.
Unfortunately, converting to the single pass requires a suitable radiator and a bit of plumbing to get the flow from both thermostat housings to the same side and teed together. Not a lot of room in the XJ-S engine compartment for such shenanigans.
John’s Cars offers a great radiator for use with big block Chevy engines, designed to fit the XJ-S. Unfortunately, if you mention you’ll be using it with the Jaguar V12 engine, they’ll refuse to sell it to you. That’s the kind of shop they are. So, most have opted for a custom aluminum rad. Supposedly some Mustang or other Ford radiators will fit pretty well, though.