I believe it is possible but you can make life harder with certain bottom ends - i.e Mondeo bottom ends have different sumps, water pumps etc which have to be changed.

It is also possible to have an 1800 Sierra CVH bottom end with a 1600 CVH head but I think the Sierra engine is built for economy rather than perfromance so it may not be worthwhile.

Hope this helps mate - they were possibilties I thought of for my XR!

CheeRS