easiest way to get the box off with the engine in situ is to drop front cross member bolts on the box side and undo the gearbox mount, driveshafts, take speedo cable out, undo the starter bolts and be aware the engine mount at the front goes through the starter, then all the box bolts and going slowly start to lower the front cross member at that corner and with a bit of wiggling the box should pop off and clear the chassis leg, common sense is also a major factor whilst doing this, make sure all the relevant parts are supported, i do think it's likely to be the wrong clutch though, check the clutch on the splines of the box shaft for fit
just a quick thought, is your clutch cable manual adjust or self adjust, if self adjust did you refit it the correct way as these can cause issues with gears if not done right, the correct fitting is using the white plastic loop thing on the cable
