I've been attempting this job on mine today and much of the previous posts rings all too true. The existence of this thread does at least suggest that it is possible, which at times I have doubted earlier today - even now I have the pump off but not 'out', if that makes sense, but have come indoors to save health and temper. The 'little difficulty' cited in the Haynes is a masterpiece of understatement.
The job has, in fairness, been prolonged by not being able to undo stuff that has been untouched in 35 odd years ( block drain plug, splashguard bolts, pulley bolts) and me not being a mechanic.
I assume you have to get the rear rail studs out of the old pump to be able to manouvre the thing out?
Mark