You'd be fairly close with your guesstimation of 13:1 with 64cc heads. You can mill quite a bit of material off these pistons to reduce compression ratio. If you get them cheap this maybe an option. The other problem with such a large lump on top of the piston is flame propogation. A smaller bump is much more preferable.
I bought a set of those TRW L2250's back in the late 70's real cheap because of the high compression. I milled off about .250" thou. and with a .035" gasket under 68-70cc heads I had a nice 10.5-11.0:1 compression ratio. Super grade fuel was still readily available then. The motor was a 327 + .030". I was using a Chevy "Pink" (LT1) grind solid lifter cam and from memory had about 160psi compression pressure with this combo. That motor is still seeing weekend use in a monaro cruising Adelaides streets to this day.
Depending on your cam selection you may be able to get away with a higher compression ratio but for a streeter about 10:1 is max these days.
Hope this helps.......Barry.