Go for the GTB secondhand if you can get 1, i used to run 1 in my b44 & it fits in fine, also if you are going to run a 4.5t in it make sure the diffs in both gearbox's are correctly shimmed or it will eat your gears, when you put the diff into the gearbox there should be no sideways movement, if there is add extra shims on the diff gear side to ensure a nice mesh with the other gear
Have fun
