Hi, does anyone know if series 3 E type seats can be converted from manual to electrical adjustment? Alternatively, could XJS or XJ40 seats be made to fit?

 

Boy, that's an interesting notion. Of course, anything can be made to fit anything else, but the E-type seat opening between the rocker channel and transmission/drive-shaft tunnel is quite narrow. You may have better luck trying to fine a Recaro seat from something that can be made to fit.
Maybe look at a Miata seat, or Porsche. Small cars are perhaps the best bet. I think XJS, and certainly XJ40 are probably too wide.

 

So I tried to do this with my 1970 using a set of recaro seats similar to what I'd put in my XKR. Fitment is a huge issue, standard aftermarket seats don't fit. I also tried to put series III seats in a series I and while I got the seats to fit, the sliders were messed up (I didn't want to drill new holes in the floor and reinforce them). I'm sure it can be done with some kind of custom fitment but I ended up with non-electric seats from a company that specialized in custom seats for hot rods and those fit great but that company didn't have an electric option (they are heated though). Here are the seats I now have in the car.

 

Finding narrow seats is difficult. Measure up the seats from a SAAB 9-3, they should just fill the space. Adapting the mounts and electronics, you're on your own.