The cost isn't $100,000.00 for an Allison 250, it's more like $25-30K unless inflation has tripled the price since a friend put one in his jet boat. If you're looking to spend $5K or less, then yes...forget it.
As far as "streetable" that depends on your taste and definition. A lot of people don't think the power train in my Cobra is streetable and stare at me like I'm out of my mind, yet I don't have a problem with it.
I'd like to build an Allison 250 powered Pantera. Someday if time and space allows, I just may.
I think it would be just way too cool to pull up with a screaming turbine under the hood.