Well, As far as I know, A toploader is a Toploader... except for bigblock toploaders which have a beefier input shaft.... Although I believe there are differences in the shifter between XY and XB... but that doesn't affect anything but the shifter and where it pops up in the car (I could be wrong ,this is from what I've observed shifter wise...)
It would make sense to rebuild the one in the shed and fit it once its built.... means maybe a day off the road vs time to rebuild....
Anyone round Brizzy with a toploader & bits to convert from a C4 for sale (or swap for any of the bit i've got for sale?)?