knock from gearbox when cranking

Recently bought a non-runner series 3 fitted with a rover v8.The engine was siezed.Replaced the engine,but now when i crank it over there is a knocking sound coming from the gearbox.
I fitted the correct spiggot bush from the first engines flywheel,tried it on the shaft and it fitted perfectly.new release bearing fitted.
The gearbox seems like an origional 4 speed.The previous owner claims there was nothing wrong with the gearbox. 4 speed.It siezed in a wood -took in water,so it had to be towed out,and they may have damaged it then.Any ideas Please

does all the gearbox move freely etc can you move the props etc as it sounds like water did damage it or the clutch is not properly aligned

Thanks for your reply.engine is back together with gearbox.if the clutch was out of line it wouldnt bolt up...i think.The input shaft should centre on the spiggot bush.when i rotate the engine using the starter a knock comes from the gearbox/bell housing even with clutch engaged.i havent started the engine yet.how could a knock come from a gearbox just by rotating it.

missing teeth on the ring gear

No i had engine out.i had to swap fly wheel from old engine.it bolted to crank squarely.no teeth were missing.the clutch plate was centered up.spiggot bush checked for fit on gearbox input shaft.the noise is definately from the gearbox.and its a knock like something hitting off the casing inside.im on hols at the moment so when i get back ill have to pull it out-something is rotating in the gearbox and hitting the side casing of the box.

Does it do it if you crank with the gearbox in gear and the wheels chocked but the clutch depressed?

Yes .when gearbox is in gear and clutch pedal pressed the noise is present or if in neutral with pedal pressed or not (depressed)the knocking is still present.this is a v8 onto a series grarbox.there is an adaptor plate made of alloy bolting the 2 together is it possible the gearbox input shaft is fitting correctly through the clutch plate but not long enough to reach the bushing on the fly wheel.i havent started the engine as i know there is something wrong somewhere.the gearbox bolted up to the engine ok-no gaps

It sounds to me like you need to take it to pieces and measure it all up and take a lot of good pictures. Has it ever run or is this a first go?

The previous owner had it 2years.used it for off roading only.its still on english number plates.he tried passing a pond in it but it got too deep and the v8 took in water and siezed.he bought a second engine for it and started to strip the siezed engine.he then abandoned the project.i made an offer on the jeep with the replacement engine included.i removed the old,swapped engine mounts,manifolds ,fly wheel,starter etc.fitted a new release bearing as the old 1 was siezed.and fitted the engine,turned it over with the starter and knocking appears from the gearbox.ill drop the gearbox next week when i get back.

yesterday i started the engine.drove it around and able to get all gears.when i stopped engine and restarted the knock was still evident from bellhousing.I think ill drive it until it gets worse.