Magnum carb fuel system in early Abody.

The stock 5/16" fuel line size will have no problem supporting the power level your stock 5.2 Magnum will make.
I assume the eccentric issues you are referring to is the short snout adapter needed when installing the LA fuel pump eccentric on a Magnum short snout camshaft. These also work very well. Most people talk bad about it without ever actually using one. Just make sure everything fits well. Sometimes you need to file the key or extension a little, same as fitting any aftermarket part. Then, during final assembly make sure the camshaft and bolt threads are clean and dry, assemble with a little red Loctite, and be sure to properly torque the bolt.