My suggestion is to see if you can get a complete car with a manual transmission in it already. That way you know all the little bits are there and you have spares to play with
Secondly, I have been told that the diff ratios are different however if you leave the old one in there then youll probably just have better highway gears.
If your car that your planning to do the conversion too has a coloum shift auto and no floor plate and you want to put a floor shift in, then forget it. I did it and it was more hassle than it was worth.
My gearbox (single rail) cost me $400.00 but then I had to buy a cluch and pressure plate, Peddal box and throwout bearing. Ended up costing me about $600.00 with alot of stuffing around. You could probably get a wreck for less if you looked around.
Good luck - Im just about to do this exact converstion again but with parts only. (You'd think I would have learnt the first time...)
Your thread got me inspired and I just done my conversion on one of my XFs. I scored a 5 speed box from a 3.3L XF wagon with bits however I forgot to get the yoke which is currently in Ocean Shores. (Bloody things different compared to the auto spline) Just goes to show you should have all the bits on hand just in case all parts dont fit.