Mushy - In NSW you must use an engine from a compliant vehicle no more then 3 years old (at the time your new car is presented for rego). That rules out the popular (in qld)older lexus motors. You would not be able to get a shelby or other BB registered at all (unless you knew someone!)
In NSW at present I would guess 9 out of 10 recent completions or in construction cars are using a gen3\4 series engine. . Heres a graphic that may give you an idea of the size of the boss motor:
You will need to have very skinny feet.
