Maybe late to the party but the seats are basically the same design except that 420s have ventilated pleat panels rather than the plain ones in an S Type. There is no difference between manual and auto as far as seats are concerned and they should be a straight swop. Bear in mind this is an opinion ...

Not much help for you on the gearbox questions but the oil pressure sender (for the gauge) is mounted on top of the oil filter housing. Yes, you can change them at home but it's a struggle. Replacements are notoriously unreliable but a straight swop. I converted my 1966 S Type to a direct capillary ...

As far as I'm aware Wellseal formulation remains the same now as it ever was. It certainly works as I've used it when rebuilding a Rolls-Royce FB40 engine.

Rolls-Royce (when it was Rolls-Royce and not some subsidiary of a certain German manufacturer) always stated/recommended Wellseal for use on their gaskets.

Not very useful information for Jaguar purposes but just as bit of extra back-ground on sealants!

Ok, I'm late to the party and you may have resolved the issue by now, but.. My experience with the Model 8 was on the VdP 4 Litre R and I had the same thing after it was 'tuned' at a local garage. The kick-down/throttle sensor cable had been vastly shortened by them (why?) and 3rd wasn't for engagin...