Classic Broken camshaft!!

Hi, put the engine back together after changing the head gaskets, something I have done before on a few other V8s in the past with no problems, however this time, it started, ran then a loud crack and was not running happily at all, and a lot of oil leaking out the bell housing so shut it down quickly. Oil level still above min.
On pulling it all apart again I noticed the rockers on the rear two cylinders were loose, which was odd, so tool the inlet off and the valley gasket, and find that the camshaft has sheared at the penultimate journal and that the end has pushed the core plug out into the bell housing (the final journal is no longer in it's bearing but flywheel side of the block).
So can I get the engine out without disturbing the air conditioning?
Is the gearbox cooler in the radiator?
If the block is toast, I do have a 4.6 which is in decent shape, how much work would it be to fit? (It does not have a distributor)

The block must be toast after that Min new camshaft bearings , valves on both sides and possible bent rods or damaged small ends and big ends. The aircon must come off as the engine needs the space at the front to get out. Thats only around £50 to regas and if you do it at quick fit and has a leak you dont pay and they tell you where its leaking

ive just re read your post and you may be lucky where it broke the least would be a new cam bearings to make sure all is true.

Valves are fine, they will have shut as soon as the cam broke (all 4 push rods were loose, the tappets dropped into area where the cam should be and were just sitting there) 3 of the 4 pushrods appear straight when rolled across a surface, as I took them out, One tappet dropped straight into the sump as I took the rocker shafts off, along with the pushrod this one I have yet to test
It's still a mystery as to why it broke, there is no sign of anything untoward apart from the broken cam, It was turned over with no ignition and no plugs till the oil light went out, then checked to make sure the rockers were getting oil, covers put back on then the plenum fitted along with the plugs and ignition leads and started after that (as I have done with many SD1s before)

If your happy replacing the cam shaft you will see if any valves are bent through the intake All the Rads will have to come off the front to replace camshaft and drop the sump to collect all the bits that dropped in. Not much more work in dropping in a good block

Just filled both the effected inlet tracts with antifreeze, no leaks, so I think at least the two inlet valves are fine. My 4.6 is just a short block, and is from a distributor less engine. I assume this will need a cam and the front cover from the original? Will the engine management cope? Can I use a standard 3.9 cam with the 4.6?

4.0 and 4.6 have different cams. You will need a cam with a tooth drive on it for a distributor You might as well lift the engine out as you have to strip that much off to get a new camshaft in. Take it to pieces and check it all over. Remember you are the master and the engine is the slave.
Its very basic when you start pulling the thing apart and theres so many videos on how to strip and rebuild a rover v8 engine. all the shells and gaskets are cheap as chips

The 3.9 block is scrap, looks like the cause was a broken con rod, there's a chunk taken out of the bottom of both bores you can see when you look down the area where the missing section of cam should have been. Now I have the water pump off and the front cover I am pretty sure it can be lifted out without taking the radiator off or disconnecting the AC as I have nearly a foot of clearance which wont be needed at all if I can unbolt the torque convertor before taking the engine off the gearbox. The car is one of the last softdash classics with the serpentine fan belt

The 4.6 I have is a Turner short block with 5000 odd miles on it from a crashed P38a. I assume I will have to fit a cam with a distributor drive and the front cover from the 3.9. Do I need to swap the driveplate/flywheel over or will the one that is on the 4.6 be fine with the torque convertor on the classic? The only other thing I think will be an issue will be the fueling.. I am NOT paying silly money for a chip!