Andrew, totally forget about the Ashcroft setup. I have driven that and it is really sweet.
As for the length of the LT 77 . It has a short bell housing and is a direct bolt in to the 2.25 and 2.5 gas an 2.5 NA 2.5 turbo diesel.
The R380 has a longer longer bell housing because LR moved the 300 Tdi forward in the engine bay. You can fit a shorter bell housing to a R380. Ashcroft has the parts and sell this kit as well.
If you have an old tired motor this is probably not the best route. I would suggest a rebuild before going down this road. 30 or 40 over pistons and a 2.5 cam make it a really nice driver. The down side is I never got more than 18 mpg highway and usually 15mpg around town. Usually less, as I have a heavy foot
I had to install an extra tank to have any kind of range.