Building a custom manifold to use quad carbs of a bike could be an option if you can weld. Adding a plunum on the inake side of the carbies will tend to reduce the over all peak HP of a tuned system, bring it down the rpm range, spread it out and add a second peak lower down. The smaller the plennum the more this effect, the other extreme of course is no plunum or infinite volume.

If you're not doing anything else forget the springs, you only upgrade them if you're having trouble closing your valves.

If you're on a budget start by port matching, deburring, reducing any restriction on the intake side (air filter etc.) and ducting up cold air induction. That'll make the most of what you've already got.