Enjoy the search - isnt this half the fun anyway?
I think the reason that you see engine swaps in the States is due to the fact that lets say, you have an engine failure or a high mileage engine in need of a rebuild, it's much cheaper in the US to swap in a small block for example. Now back in the UK it just doesnt happen, obvioulsy due to the lack of a small V8. You have t remember that in the UK the jag engine is a large capacity engine, and there is no cheaper alternative that gives the same power. I am also certain that these replacement engines were only installed when the cars had little value. Now with number 1 e types selling for $100k + there is no reason to install a Chevy small block!
Looks like Joe has alot of experience with the E-Type and is right about the better years. The 3.8 was a free reving engine, the purists choice, but also the most uncomfortable and no synco on 1st. The 4.2 cars have more torque, much better seats, all syncro box and as Joe mentions, they still have the lovely toggle switches!
I personally dont care for the uncovered headlights on the later models, and I think the V12, although another superb engine, just dissapoints with the heavy styling.