What is the rest of the engine combo? It sounds like you've got a stock '67 383 2 barrel and just changed the cam and timing set. What cam? Speed Pro CS327?

If you've got a two barrel on your 383 (which is a big block) it probably wont spin much past 4000 rpm and keep making power, its a tiny carb. Don't get too caught up in the carb calculators, especially since they are geared for four barrel carbs which are actually rated differently. If I remember right when they flow test a two barrel they do it with 3 inches of vacuum, whereas a four barrel is flowed with 1.5 inches. So basically a 500 cfm two barrel is probably comparable to a 350-400 cfm four barrel.

I would recommend a four barrel on any big block, you don't have to go crazy. You wont suddenly have a race engine, but a factory four barrel intake with a Carter or factory Holley four barrel will give you some extra punch.

As far as the difference, if it revved out higher before it could be a couple things. For one the cam is almost guaranteed to not be the same specs, and the old one was worn. It is more likely that the new cam is bigger, so you should actually have more high rpm power. The old stretched out timing set would effectively retard the cam and may have shifted the powerband up. Its also possible that your new cam is too far advanced if you just installed it by lining up the dots. Does it have more low end torque?

You may also be encountering some valve float if the new cam is more aggressive and you're using stock springs.