Did a Mass-Flo EFI installation last winter on my 428 FE and got a season on it this summer (about 7,000 miles). Very pleased with the driveability of the setup and it's performed without a hitch from sea level to 14,000 feet.
Put the car on a chasis dyno before I took the Holly off and after I put the EFI on. No significant changes in power or torque but the curves for the EFI were considerably smoother.
Minor early teething problems but they were immediately and correctly addressed by Chris at Mass-Flo. Very easy to talk to and knows his stuff.
Looks pretty much like a carb but with fuel rails. Very complete kit, nothing extra needed.
DonC