I pieced together an EFI setup on my 65FB. I am using a Microsquirt controller and an EFI FAST throttle body I got on flea bay. I still had to add a crank sensor, coolant sensor and wideband O2 sensor in addition to the sensors already in the throttle body (MAP, TPS and IAT). I also swapped in an EFI tank and ran new fuel supply and return lines. It's a lot of work and it adds up.
The Microsquirt is not plug and play. It requires quite a bit of initial tuning. However, you have 100% control over all aspects of the tune, including ignition timing (I am running wasted spark with no distributor). I now have it pretty well dialed in and it is amazing. It starts right up no matter the temperature outside. No more permanent gas smell in the garage. Finally, it runs like a bat out of hell. While I am sure that my carb could have been tuned for the same high end power, there is really no way to tune it for the same overall drivability. You basically get to set the AFR and timing for any condition your car will encounter. This means I can get a stable low and cool idle, terrific throttle response, great cruise millage and super high output performance. I also dumped my dual plane intake for a single plane and it works great. I do not think I would get the same idle and low RPM response with a carb and the single plane intake that I do with EFI.
That said, I am not willing to put a comb over modern factory type efi on my car. When you pop the hood on my 65, it looks like it should until you notice that the distributor is missing and there is a fuel pressure regulator. Still, those differences are subtle and do not distract from the overall look and feel. Also, my car had a modern (circa 1992) drive train (302 roller and T5). If it had been the original 289 and 4 speed Top Loader, I would have been more hesitant. But, as they say, it's only original once and that ship sailed before my car ever came my way.
So I say, unless you want to maintain originality, there is no reason not to go efi.
1965 A Code GT Fastback