[swedishandgerman]

I often contemplate this because a wider gap provides an improved way of opening and closing the valves more instantly which gives better sealing on the compression stroke. On the other hand, a small gap allows the cam follower to "follow" the cam profile better and the exhaust valve is open for longer allowing more time for the exhaust gases to get out.

All said, the recommended gaps in the book are calculated on the ramp of the cam to achieve the optimum opening and closing of the valves as well as time duration. So for an a-cam it's the narrower gap because the cam lobes aren't as tall, then the valves are open long enough to give proper inlet and outlet of gases. For a b-cam it's the wide gap (taller cam lobes). For a fuel injection cam, it's the narrow gap - I guess so that the valves are open for longer. If you accidentally use 0.043 setting you've found somewhere, it just won't run properly.

As such, you need to find out what cam is in there, rather than what engine you have. IE: If it's a B20A fitted with a b-cam, it's the wide settings. As above, a 1970 1800S shouldn't have a B20A!