I would agree entirely with the comments above about fuel supply; check for fuel tank venting (remove cap and run engine), blocked pipes or collapsed hoses.

Then I would check the coil, as mentioned above. In fact, if it is very old consider changing it anyway, they tend to fail by going open circuit as they get warm and being ok again when cool. In any case, they do degrade over the years and a new one can really sharpen the performance of your engine. I have found this on several old vehicles. When changing the coil, also change the capacitor (condenser) in the distributor. Both items are really cheap and well worth replacing.

Not related to this problem, but check your HT leads too. These can break down under pressure (high engine loads), but this will show as a misfire when climbing a hill.

Adam..

The picture shows a block with serious corrosion in the water jacket. It may have been run for a number of years with no antifreeze (and therefore no corrosion inhibitor). Whilst that looks eminently repairable, other areas of the water jacket may be thin as well.

In this case, the white smoke would have come from water leaking into the hot exhaust port and evaporating. But the original question was about white smoke on the overun only. This could be a leaking headgasket, or even a cracked head.

Compression pressure is higher than water jacket pressure so any leak is usually into the water jacket (unless the gasket has blown away completely). Therefore the radiator header tank is the place to check for compression gases or oily residue. The ideal test is a cylinder leakage test, which tests the stationary engine by charging the cylinder with compressed air, but you may not have the kit for this.

My advice would be to run the car locally and keep an eye on it. If it clears up then fine, if it is a head gasket, it will get worse, so the problem and water consumption will get worse. At this stage, act quickly to avoid damage to pistons and bores from water ingress.

Hope this helps
Adam..