I'd keep the 3.08 gears for highway driving and engine longevity. Also keep in mind, that depending on trap speed, you can actually run a faster 1/4 mile, if it allows you to avoid shifting into 3rd gear. Food for thought. And, even if you change to a deeper gear, you should still change to a torque converter with the proper stall.

Also with the factory compression ratio and assuming a factory ground (116 LSA) you should have no problem running super unleaded.

The vacuum does sound low with that cam even at 600rpm. It's not a large cam, and with a 116lsa, it should be pretty good. A tighter LSA might give you a little more low to mid-range torque to go with those 3.08 gears, but again, at the sacrifice of idle vacuum and octane tolerance.

First thing I'd do is properly set your dwell, and then check your initial timing and total timing (with vacuum port on the carb plugged). Also record when the max timing comes in. The 670 heads are closed camber, which tend to like up to 42° total timing. I'd try increasing it by 2° increments until you get to that point and see when you get the best performance. Don't bother with "light" aftermarket springs and weights. They can cause the distributor to hang up, and artifically starting the advance earlier can tend to cause irradic idle. Then verify with the engine warmed up, if you have excessive starter load. If you do, you might have to compromise and back off you timing. Once you get the timing set, readjust your idle mixture for highest rpm (you'll probably need to adjust it leaner than it was before. That's because more timing can cause enrichment. Then readjust your idle speed back to 600rpm. I think you'll find a significant vacuum increase. If you're not getting at least 14 Hg-in at that point, you should redegree your camshaft to make sure it's set properly. You might find you need to adjust your cruise mixture leaner than before as well.

Also keep in mind, too tight a torque converter can also cause low vacuum. So be sure to order the new torque converter once you have your timing and carb mixture reset.