Here's a pic of an SR20DET. It's in a 510 engine compartment, but gives you a good idea of where the parts are relative to the brake master cylinder and clutch master cylinder:



SR20DET in LHD Datsun 510

Reportedly an SR20 fits into a 1200 without cutting the firewall, but you'd probably need an automatic car to avoid modifying the manual-trans tunnel.

The KA24 fits better, and has more capacity too. You might consider that.



KA24E in LHD Datsun 1200

Then there is the L20B and Z20 swaps:



Z20 in LHD Datsun 1200

Engine swap article