Not sure about the heads but Clean the block with it all together. You dont want to accidentally get stuff in the cooling system.

 

I agree, don’t clean it with it partially disassembled.

 

Shipwrights' disease?

Goes something like this:

Sailor owns boat.
Boat has burned out light in galley.
Sailor decides to change bulb.
Sailor notices socket is corroded, decides to change socket.
Sailor notices wiring frayed while trying to change socket.
Sailor decides to change wiring.
Sailor notices galley ceiling slats are rotted while changing the wire.
Sailor decides galley ceiling slats need changing.
Sailor notices ...

this goes on and on and on and on and pretty soon, Sailor is undertaking a
major renovation of his boat because of a burned out lightbulb.

Hey Sailor! Fix one -ish problem at a time. Don't worry about the oil covered engine, it's a Rover and it's going to leak. Sorry, but this is a fact. Yes I'm sure there are 2 dudes on here who rebuilt their motor and now it does not leak. Great!
Fix the head gasket, replace valve cover gaskets, get it together and run it.

Trust me you'll have plenty of time to tinker on this as you go!
I would not try and clean it, and for the love of god don't put oven cleaner on any part of your car (that you like).

Keep at it! Good on you for taking up the challenge!