You got it. Finding the right propeller is not scientific at all.

I also think the 9x7 prop has a bit too much pitch, though a wood prop doesn't "dig" quite as hard as an APC.

Those older guys are used to running older cross-scavenged engines that produce a lot more torque than today's typical sport engine. The engines ran at lower RPMs, and could swing higher-pitched props without straining. Of course, they used a lot more fuel. I have an old Enya .19 on a combat plane that swings a 9x4 prop at 12,500, but uses over an ounce of fuel a minute.

A student of mine had a .25LA on a PT-20 trainer last summer. He started out with a 9x6 Master Paintstick. Frankly, with the 9x6, I thought the engine sounded sick. It wouldn't pull the plane out of the grass, and we had more noseovers than takeoffs. I had him try one of the 9x4 props I use for combat. With that prop, the engine spun up to a nice RPM, and yanked his plane off the ground like it weighed nothing.

The bottom line is, it doesn't hurt to experiment.