Hello old53 and welcome to the forum!
If the A7 head had been fitted to your engine without either changing the pistons or modifying (enlarging) the combustion chamber, this had very likely been the cause for your broken cylinders. If an A10 engine with a geometric compression ratio of 8.5:1 is fitted with the smaller diameter A7 head without further modification the CR will rise to approx. 10.7:1.
Ref your questions:
1. Hard to give an exact number for valve to guide clearance. This is a matter of feeling and experience, moving/rocking the valve in the guide and judging whether the play is still ok or too much. If you are in doubt, it is best to show it to someone who is more experienced.
2. Currently don't have the brilliant Roy Bacon book at hand, but I'm sure someone else can look the original carb setup up for you. If not, I can do so this weekend.
3. Needle and main jets are fully interchangeable. Some Concenrtics do have a removable pilot jet, others don't, depending on the year they were manufactured. From my experience the main jet can be chosen one or two sizes smaller when using a Concentric (compared to a Monoblock of the same size).
Hope that helped a bit, Markus
edit: Looks like Bill was faster on the CR issue