if i were to block off the waste gate...
 

my question is this. if i was to buy a turbo manifold with a external wastegate opening and i have a internal wastegate on my turbo . by blocking it of on the manifold would it affect any performance?

Why would you block it off?

externals are better and internals are used more on factory applications because they save space....get a different turbo outlet, if possible, or modify it instead...use the external if you can....the outlet of the turbo usually houses the wastegate, and if you could just swap it out or rig it where it isn't used, then you would be better off

If you really wanted to you could route your dumppipe off the wastegate to the exhaust.