The HC bay takes v8;s and is big enough for most 4 cylinder blocks - so in reality any combination is possible - just need a decent engine / box set up and way to go
Cant see any reason why a Carlton engine / box would not be any good as a converrsion.
Obviously helps if you are good at fabrication and modding things
May also need to make tunnel / bulkhead mods .
Im currently fitting a Suzuki Baleno 1600 16 valve fuel injected engine to my firenza - mated to a Suzuki SJ413 5 speed box / purpose prop then to a Magnum 1800 axle .
Luckily i bought the whole Baleno for the conversion as i had to cut out the drivers bulkhead in order to use the Baleno servo / master and clutch ( as well as complete pedal box) The Inlet manifold wouldnt allow the use of original servo ( no damn room) .
I would advise you get a complete donor car as you never know what you might need to complete the conversion.
Jon