How to know what modifications have been done to 318?

Your intake and carb choice is solid. I wouldn't worry about the heads right now if the engine is tight and not using oil. As far as the cam goes, i doubt it's anything huge. A vacuum reading of say 13in. or higher would be in line with the type of performance your after. You didn't say if you had headers and a good 2 1/4 or 2 1/2 exhaust, but that would go a long way towards what your after. Depending on your rear gear and torque converter you could have a fun package that has good street manners and will run pretty strong. Just remember when you get it together that timing and carb adjustments will pull the package together.