Boring out a block increases compression ratio. When we bore a motor out 0.060" (1.2mm) we usually take about 0.010" (0.25mm) off the top of the piston (on standard motors).
If I were to put a new head gasket on my motor I would at least get the head cleaned up, the block should be alright, maybe just need a scrape to get the old gasket off. I know on the machine at work we can take as little as 0.001" (.04mm) off heads, assuming it's dead straight....** Then I'd also get new or ARP head bolts, since your going to the effort any ways.
**more than likely the mininum we could take off is 0.003 - 0.010" which in the long run is fuck all.
I'll take my silvertop head to work and see.
