Running problems

What type to fuel system are you running? EEC-IV speed density, EEC-V mass air? Or is it carbed with the duraspark system?

Double check your firing order. It's not 15426378 like the old 302 and 289 motors. The firing order for the newer 302's as well as all 351 motors is 13726548. Running the old firing order will cause the symptoms you describe.

Let us know.

Double check your ignition timing and dwell. Make sure the advance is hooked up correctly.

What type of carb are you running? Anything lower than 300 cfm and it just won't give a 351 everything it needs to push that car around. I'm assuming you have something around the range of 500 to 650 cfm. Double check the tuning of the mixture adjustment screws. Check the position of the fast and low idle adjustment screws. Check for proper function of the choke.

Check for any vacuum leaks. Not only vac hoses, but such things as carb to manifold gasket, etc. A vacuum gauge will help you here. Also do a compression check just to make sure a head gasket hasn't been compromised.

From what you describe, I'm leaning towards a possible ignition related issue, or the carb is out of adjustment. I hope this helps.