Thanks for the advice, guys. I'll take a look at that option, Joey. I've got a great diff guy here in SF (strangely enough), who was going to swap the ratio but he might just have a late model XJS pumpkin around. I'll definitely have to ask. I'm going to keep the 4 speed. I have a modern turbo'd 6 cyl bimmer for 0-60 sprints and I'm not looking to do stop light drags with Mazda 3's, so the 4-speed is perfectly fine for me in this particular car. I do get the attraction of the 5-speed though.
Gas mileage? Frankly, I couldn't care less, as long as the car is running as well as it was intended. As it is, the E-type is one of the lowest carbon footprint, greenest cars on the planet. If we would have all been driving them up until now, we wouldn't have foisted the umpteen metric tons of carbon that the 5-15 cars we've purchased in the 50-60 year interim created with modern manufacturing. And don't get me started about the auto battery manufacturing process in China..
BTW-I spoke to J Farrel about various items and what a nice guy. Thanks for his number!