64 dart convert to EFI

This can be a very difficult question to answer, and Pishta has brought up the core. The answer is not simple. How much money are you willing to spend? NONE of the "bolt on" EFI systems are really all that simple to get running very very well, depending on your computer skills and electrical knowledge.

Some things to ask yourself
1...How much money am I willing to spend
2....What are may electrical and computer skills
3....How long am I going to keep this vehicle and how much, really am I going to drive it
4....If you can not install this yourself, it is going to cost a LOT of money shop time, and a UNIQUE mechanic who is able and willing to operate on old vehicles with not OEM parts

Your mechanic IS incorrect, however. There ARE systems that would work on your V8, they simply are not a "bolt and go." The Holley sniper series, the FItech which is similar to the Holley sniper IN THAT they are both a "4 bbl carb like device" with most of the electronics in the unit