The shortstar is a 3.5L dohc V6 (LX5) based on the architecture of the Northstar V8. It's 2 cylinders shorter, hence the nickname shortstar, it was used in 2nd gen Auroras and available in Intrigues. The Aurora motor (L47) is a 4.0 V8; the major difference from the 4.6 Northstars being a reduction in bore size, with some weight reduction as well.
Either way the 4.0 Aurora should put up a good fight against a Mark VIII. I've raced one once in my 95 ETC, pulled it by about 2 carlengths by 80. In comparison I pulled around 3/4 to 1 car length on a Mark VIII belonging to my friend, and 1 on my own Mark VIII with me driving the VIII and a friend driving the ETC.