Luca, I just did mine in an RV9A (so, larger panel).
Bob Nuckolls says he uses a 0.70'' separation with Klixon. So, I guess 0.70'' will be OK for heating issues.
Klixon are smaller that P&B and since I use P&B I applied the same separation formula and discovered, if I recall, a separation of 0.90''. 0.90'' is 28,8/32''. I ended spacing both W23 and W31 toggle breakers 30/32''. In this way toggles will be easy to reach without conflict with the other toggles on the sides. See my installation on my website as follows (all down on the following page)
http://nuke.rv9.it/Pannello40h/tabid/57/language/it-IT/Default.aspx
Last information I can give, P&B W23 and W31 will touch if installed at 22/32'' hole distance. Anyway, I fear that this will produce some effect on thermal characteristics.
By the way, I would like to ask a question. I mixed W23 & W31 (i.e.: alternator field push/pull breaker just beside alternator switch and starter push/pull breaker immediately after) in my panel installation. I spaced all of them 30/32'', in order to let me change my mind in future and change one for the other. While installing them, I discovered that W23 load side is the opposite of W31 load side (I can't imagine why manufacturer did such a thing), so if I want to align them on the same bus row, I will have to install W23 upside down (thus obliging me to print and apply new #s for amperage; it's ugly to see numbers upside down). I read on this forum and on Bob's FAQs that polarity is the same for breakers. A neighbour of my hangar said me that this is not true, because of the inner winding (?) or somewhat similar. I did not understand well this. Someone of you cught the problem?
Camillo
Edit: after reading Bill post, I would like to add that I also did as Bill, with a separate aluminium bar. I used the one which came with the fuselage QB.