Can anyone tell me if I can replace the clutch fork by sliding gearbox back by uncoupling between box and bell-housing.

No. Bolts holding the box to the bellhousing are only accessable from inside the bellhousing. And even if you could remove the Box from the Bellhousing without taking it from the engine how would you access the Clutch Fork etc.

You need to pull the Bellhousing from the Engine.