Phil came over and we measured things and trial fitted them and we're fairly sure it will fit. To be really sure we need to take the input shaft out so the motor can go all the way into the bell housing, like it would when the input shaft is reground to fit the motor
Removing the input shaft from the gearbox is tricky because you have to lift the layshaft with it, and to lift that you have to lift the reverse idler and more importantly, the mainshaft. To lift that you have to lift the selector rods and you can't lift them both because there is an interlock to prevent two gears being selected at the same time.
To get the selectors off the selector rods you have to drift out the roll pins arrowed in the picture. Unfortunately they're at the end of long somewhat wobbly shafts -- hammering moves the shaft more than the pin.
Apart from putting them against a suitable anvil, using some sort of puller seems to be the only sensible solution.