if your planning on dismanteling the car and building it the way you want then y not save a couple grand and do everything the right way. I would go with the 69, especially because pro-touring builds can get pricey.
I gotta go along with varlo on this - not to mention I'm also partial to '69 Darts and am currently in 'build mode' myself on one.
If the price of building an engine is scaring you off of this you have two alternatives: (1) Sell off whatever engine you decide on with the car and put the money into an already built crate motor; or (2) Pay less for the Dart less engine and put the money into a crate motor.
The nice things about crate motors are: you can choose whatever power level you want, it's brand new so you know the history, and you can probably get a warranty with it.
That's what I'd do. But it's your money - your choice - your decision.