That sucks you went through that! But, that doesn't sound quite right... I know several people locally who have installed H55Fs into FJ40's and they did not have to have a so called diesel engine clutch assembly...
Also, the "better gas mileage" condition only applies if you regularly run your cruiser at the same top
speeds with the 5-speed in 5th gear as you did with the four speed in 4th gear. Thus, your top speed RPMs will be lower and essentially using less fuel. But, the thing is, most people get the 5-speed in and run it up to the same top
RPMs as they were running the 4-speed at, which equals the same amount of gas consumption. I'm sure I'll do it when I finally get a 5-speed in my FJ60--but I'm not in it for gas-mileage, I just gotta go faster!!!

Also, first gear in the 5-speed is lower so I guess technically it would probably cause you to use more fuel as well...
And another thing, H55F 5-speeds are available BRAND NEW at the dealer for $1800-$2200 USD, so those who have split t-cases can pretty much just bolt them in. That's why I'm interested in it if he'll seperate tranny from t-case. I've already got a split case. And if you don't already have a split t-case, this setup is still a pretty good deal--especially it being already rebuilt.