You didn't mention any obvious ignition problems, so I'll assume there aren't any present. The factory HEI has plenty of jolt, and I wouldn't replace it just yet. The MSD units are great, and I've used them in turbo applications where the cylinder pressures call for more voltage at the plug. If you insist on an ignition upgrade and your current distributor is in decent shape, you can buy the 6A box for it. I believe MSD still makes a plug-in harness to connect it to the external coil HEI too. I'm digressing. Leave your HEI alone right now if it is in good shape. The carb & manifold will give you a noticable performance gain, IMO, but don't go larger than the 600cfm carb for an otherwise stock 305.
Chhers, MIke V
