Fly the plane with the 8x3, since that's what you'll be using in combat. The airspeed and thrust changes can mess with how tightly you can turn, so you don't want to set your elevator rates based on one prop, then fly with another.

Yeah, those Magnums are actually more powerful than the LAs. But they are designed to turn smaller props at higher RPM than the LAs are. As a result, the 8x3 is actually a little too big for them on the ground, and the engine is "lugging" and not getting in to it's power band. So they tach low on the ground compared to the LAs. The LA is already about maxed out on the ground, the 8x3 is almost pefect for it. If you put a 7x4 on the LA, it doesn't rev up much at all.

But, when the plane is in the air, the prop doesn't load the engine as much. The LA is already topped out, but the Mag can unload and get in to it's power band and I think pulls harder in the air, espeically if you have an airframe that isn't too bad in the drag department. So while the LA might turn an 8x3 at higher RPM on the ground, in the air, I think the old Mag unloads more. However, this also means that you need to be sure not to launch lean, since the unloading will cause the engine to need more fuel. If you can't hear an RPM change when you pinch the fuel line on the ground, the Mag is too lean, and while it's peaked on the ground, it will sag in the air. I see a lot of guys get this wrong with the Mag. A couple hundred RPM off peak is probably not enough, you need 500 or more, though I do it by ear, not with a tach.

The new Mags are set up to turn the larger 8x3 prop on the gound better than the old ones. So they tach higher, but I don't think they unload as much.

I think my dream SSC engine would be an old mag design with a new Mag carb and the engine internals built to the old Mag sizes, but with the new Mag quality. That would blow the current engines away, I suspect.

(one of the biggest internal differences in the old and new Magnums is the way the liner is made. The old Mags have a really odd liner. The intake ports aren't normal "windows" in the liner, they are slots that go all the way down, so incoming fuel is actually touching the downward moving piston. I don't know why they did that, but I suspect it limits power compared to the more conventional designs. The new Mag has a normal looking liner. One of these days I'll pull both engines apart (again) and take some pictures of the internals).