in the battle for power, out of the in-line six. The air Intake and exhaust are like a stage one.
Stage two would be replacing the cam and the old iron head to an aluminum head with roller rockers. The aluminum head will run cooler and give you a better combustion dome that matches the upgrades you have already done.
The roller rockers will run smoother robbing you less of that fly wheel torque.
As to MFG's of cams I would stick to names you trust, Crane, Eldelbrock, Herbert. With a bigger intake and exhaust I wouldn't go to a radical loping just a longer duration. This is where that new aluminum head will shine with larger valves both intake and exhaust.
As to stage 3...stroking the main block
Stage 4..? Forced air either a blower or turbo?