Sambo,
I understand the anguish you are going though. I seriously looked at the Boss for my RCM, but engineering concessions and the availability of the motor I chose changed my mind. Unfortunately you don't get that choice. It's a tight fit but it can be done, in a RCM at least. If you have the will you will find the way.
Before you decide which motor you really need to decide which kit first. Some kits will fit Ford cammers some will need to be modded or won't.
The good thing about a Boss in a cobra is the bottom end grunt (or lack thereof) is no where near as obvious in a 1100-1200kg car compared to the 1800kg donor.
Les, with all those Ford bits lying on the floor did you consider using a 4.0 six (they're cheap) for rego and swap later on? (oops, I didn't say that!
)
Cheers