It makes sense for me to consider because of the V8, but certainly I'd dread the idea of it hooked up to the 2.5NA. [If anything that needs an underdrive the way it's powered right now!]
You'll find varous posts by people who have fited the Ashcroft and others who have overdrives. The Ashcroft works well but it still leaves you with a 4 speed box (or 8 as you have low range) but the OD gives you a 6 speed box as you get OD3 and OD4 and you still have low range. I use the 4 to 4thOD change a lot as it is just around 40-45 mph which covers a lot of A road driving..