Wrecking an XF efi is not a bad idea if you have the space, time and means of disposing of the shell. I wrecked a 351 ZH once, it took me a week to remove everything, and cost me $50 to hire a trailer to get it to a wrecker. So i ended up getting a 351, C10, 9inch LSD, & a/c , PS for about $1000 and a weeks work. With someone who knows what their doing figure on a weekend for your manual conversion. I find these things are best started on friday night, gives you plenty of time. The beauty of wrecking a whole car is that it ensures that you have all the bits you need, and you can see how it all goes together.
You are right about the auto vs manual. Ive changed 3 BW autos in 5 years, but thats probably my driving style. Manual is much more hassle free.
Ohh, i forgot the speedo cable. you will probably need to change the speedo driven gear, Im not sure if the bw auto one will mesh with the manual boxes gear. And you will definately have to change it if you change the diff.
with diff gears, higher ratio means you take off faster, but the compromise is your motor revs harder. gears typically range from 2.75 (low acceleration, typical auto gears) to 3.5 (found in falcon utes and panelvans with the 200 ci motor and manual) and can go up to 4.11.
I currently run 2.75s behind my toploader (wide ratio) and its pretty sluggish off the line. the difference between 2.75 and 3.00 gears is quite noticeable. If you wreck a XF. swap the diff too.