6 cylinder gearbox question

I have a 6 cylinder 2a, will a gearbox from a series 3 4cyl fit?

I realise i'll have to swap bell housings etc but is there anything else i have to change?

Don't you have a IIa gearbox already? I'd definitely rebuild that instead of fitting a SIII gearbox as the IIA gearbox is stronger. The boxes are usually cheap and easy to rebuild. I know that 4cyl SIII bellhousings can be fitted to later IIA gearboxes, but you'll need to dismantle the gearbox to some extent to do it. I think earlier SIIA gearboxes have a different front layshaft bearing or something. You could have a scan through the SIII parts catalogue to check if there are any other differences but I don't think there are.

stage1v8.net - Manuals

the reason im considering a series III box is for the synchro, I'm quite happy to rebuild it and swap shafts etc?
is the II box much stronger than the III?
where is the common point of failure on the III box?

I don't know how much stronger the late IIA box is compared to a SIII box, but I've many times read articles in LRO etc which go on about how superior the IIA box is to the SIII. The only breakage I've had on a SIII gearbox was an input shaft bearing, but that bearing is identical to the IIA one. A significant difference between the two gearboxes is the layshaft. They have the same size bearings but the SIII layshaft is a one piece design whereas the IIA layshaft is splined and all the gear wheels slide onto it. I would imagine that the the latter design is stronger because the grain of the metal in the shaft and the gearwheels can be ideal and there would be less stress risers. I would guess that the grain of the metal in the SIII shaft are a compromise and that the design was introduced as a cost cutting measure. I think a SIII layshaft is very expensive because of it's complexity. If you strip the teeth off the first gear on the layshaft on a IIA (I've seen that) then you can just slide a new one on for probably less than £10.