Backpressure is not good. I'm not going to get into a whole essay on exhaust velocity and scavenging, but in your case, HP manifolds are better than log manifolds and headers are a step better yet.

Until you pull a cylinder head off and measure how far down in the hole your pistons are, as well as any dish or valve reliefs, we are guessing at your compression ratio, which isn't a good way to pick a cam.