A 9/34 pinion/ring gear gives you a very common 3.78 gear ratio, not in overdrive. Based on your speed versus rpm, I calculate your overdrive at about 36%. That is an effective gear ratio of 2.78, which is pretty tall for a weak flathead. How fast will it go not in overdrive? Out of overdrive (assuming 36% overdrive), you should get 65 mph at about 3100 rpm. It would be a bit noisy and uneconomical to cruise at that rpm and higher.
You have three choices.
1) Change rear end gears from 3.78 to 4.11; that would get your effective gear ration in overdrive to 3.02. You might get a little more speed out of that lame engine.
2) Change the overdrive from 35% to something lower, maybe about 26%. That also gets you to an effective gear ratio of 3.00, and maybe a little more speed.
3) Tune and/or build up the engine to generate more torque in the mid (2000's) rpm range. Start with a compression check; you do not want to see any cylinder under 85 psi, or a wide disparity (>15 psi) between cylinders. Check the plugs to see if the engine is running to lean (grey/whitish insulators); or differences between cylinders. If the plugs and compression looks healthy, you might try advancing the timing and increasing compression ratio. Mill the heads if you want to keep stock look.
A healthy engine should push that crate to 90+, and maybe a little higher in overdrive.
