I have and 83 el camino 305 and i bought a brand new 350 same year from Chevrolet and switched out the motors and connected everything the right way (well thought i did) should it fire right up or would i have to mod anything on my computer or electrical wise

 

That's going to be a tough question to answer without knowing what exactly computer controls were on both. I would assume you wouldn't be able to since the PCM was probably different from one to the other.

 

When I did my G-Body wagon, I didn't bother with any computer controls, not that it had many to begin with. I just took whatever I didn't use on the wiring harness and tucked it away. All it needed for the engine was the ignition, and an ignition hot wire for the electric choke, and I've yet to set up the torque converter lockup because I need to set up a kill function on the brake switch. Otherwise the carburetor, ignition timing and everything is completely analog.

 

Do you live in a State where Emissions and Computer-Controls are mandatory for a Vehicle of this age?

A simple Edelbrock or Holley Carburetor swap, with an HEI Distributor would make life much more easy for you.