is a 750cfm manual choke edelbrock carb too big for a semi built 355 small block?  

I think it depends alot on what kinda power ur wanting to make, and if your heads are capable of putting that carb to use  efficiently. I'm sure someone could give you a more accurate answer if you added info about what type of build you used in ur rebuild YAAAY new engines are fun

  But in my opinion I think it would be fine.  

It depends on what manifold your using, what cam you've got in the motor, head specs, etc.

750 cfm might work well for your 355, but I'd think something in the 600 cfm range would be better.  

its a dual plane weiand intake, vortec heads, 64cc 1.94 heads, with comp cam-268 duration, 454 lift on exhasut and intake  

With that combo, I'd definitely recommend a 650 cfm carb. You don't have a ton of lift, the displacement is small, and you've got heads with small valves.  

will it hurt the engine or performance with the 750  

It won't hurt the motor unless the carb is way out of adjustment, just performance. If you throw too much air/fuel into the engine, the motor will run rich. This means you aren't burning everything going into the cylinders. This leads to carbon build up and other bad things that hurt performance. You want your engine to use just the right amount of fuel/air into the motor for best power without detonation.  

what should i do to the carb to get the right air fuel ratio, being specific!  thanks!