You wrote that you set your timing to 16BTC with vacuum off; I think that is a little much. Too much timing advance causes high pressure high temp combustion to remain in the area of the cylinder head longer than later timing. This is a potential for overheating. You write that "it might be running a little lean, but the tail-pipe hasn't turned gray from black yet". Lean running will not turn the tail pipe gray or black. It will turn it white or not change it at all. Furthermore Weber carbs (as all other carbs) can be adjusted to different mixtures at different operating positions. Fuel should NOT be used to keep the engine cool, BUT too lean will cause the engine to run too hot. Correct mixture will give good power and close to the best economy and emissions. Best power is achieved around 0.85 lambda where best economy is around 1.1 lambda. Best emissions is around 1.0 - 1.01 lambda (and of course a catylitic converter would do it even better but not without FI). The Webers must be very finely adjusted and will struggle hard to keep mixture within reasonable limits while driving, but between 0.92 and 0.97 should keep it running fine. This will translate to about 0.5 to 1.0 %CO on a CO meter. Check it at all running conditions including highway (do this first using a portable meter). Timing probably should be somewhere between 5° and 10° BTDC at idle speed, but be very sure that it advances to about 30°-40° as the engine revs up to 3000rpm. Assuming your head gasket is not blown, you will find a big advantage with power, economy and a much cooler running engine.