Re the tappets, go to your haynes manual on page 46, there is a table of clearances. The big thing to note is the late '60-'62 A10's ran a different cam so if you have a '356 or 357 cam fitted need .008/.010 clearances. If you have more your engine will simply rattle more.
Can't help you with plugs, my A10 is an alloy head and on methanol, so not comparable.
Breaking in an engine is always another open ended question. It does depend what exactly your breaking in. For new plain bush crank and big ends (which like a gentle break in), the answer is different to rings which actually like to do some work during break in.
In my opinion the general answer is to quietly build up load on the motor over about 1000 miles, and then treat normally, but during break in to make sure the engine is given some revs, without necessarily running under heavy load. Does depend on your riding style and the clearances in the motor...varying revs is good, so B roads rather than A roads
Don't labour the motor,
if it feels like its tightening up... trust that feeling, it probably is.
Giving the motor some throttle is not a bad thing.
Sounds like it needed that pull down!
Tim