How to know what modifications have been done to 318?

You could put it on a Dyno and then some of these pros could probably tell you real close to what the build was. Very nice looking car. Any other Mods under the hood? Ignition? Coil? etc...
My opinion is that the Flat spot when under "quick" acceleration is usually Carburetor related. As stated above tune it based on Vacuum. I definitely want to Dyno My 318 for the same reasons. I have no Idea what the build was. I just know that it gets up and goes and has a mean Idle.
My Fuel was not staying primed like yours and I changed out all the Fuel hoses with proper Clamps. Problem solved. I put a 600 CFM 1406 direct from Edelbrock and My "Flatspot" is completely gone.
Any More information you can give us will help us help you.