Fred, it's great you're enjoying the new engine.The 500 cfm carb is primarily designed for dual quad set-up and it's jetted rich for use a moderately tuned 6 cylinder.The Edelbrock tuning guide that came with the carb, shows the various combination of jets and rods to change the fuel mixture in steps.Look only at leaning the "cruise" mixture one or two steps, while keeping the power mixture the same.If you lean it out to far you will experience surging or flat spots when you depress the accelerator slightly at around 35-40 mph in 3 gear direct with a fully warmed up engine. Overleaning the part throttle mixture won't hurt anything for a short term while you are testing the new jetting combination.I wouldn't put much faith into plug readings, modern fuel additives may cause the plugs to run white even with a proper mixture and onece a plug is darkened,it won't lighten properly, so you can over lean by looking at the plugs in that case.
The vibration you describe seems to be independent of engine speed so look at the drive train, driveshaft balance or perhaps it's in the tranny itself, does it vibrate in 3rd direct at 70?.I notice on the A833 in my 51 truck, there is a sensation of increased vibration in OD around 45-55 mph, but it smooths out over 60 mph.Probably just the engine power pulses felt through the engine mounts.
