A flat top piston with a .040 quench height and 58cc chambers makes 9.7:1 compression. That's decent, but may not work too well with the factory 30-30 cam.
IMO It would be OK though with a cam that has less advertised duration (like 270* at .004 for example) but you will probably be way off the factory power numbers with less cam and over a point less compression.
If I had to guess, to take a 64cc head down to 58cc you are talking about milling somewhere around .050-.060" off (I think) Definately talk to a machine shop about this, because when you start cutting that much off heads, you start creating other problems, like valve geometry (pushrods "act" too long), intake manifold fit, exhaust fit (probably not a big deal), etc.
What about using longer conn rods and a 383 type small dome piston? Like a 6.125" rod and a 1.420" compression height piston? You'd have to use a thick headgasket, but it might be cheaper and a better setup than factory rods and pistons.
I was thinking about the Wiseco #K237A3 (.350 hollow dome, 1/16" rings, +12cc piston). The piston would be .020 out of the hole on a 9.025" deck block which would mean you need a head gasket that is .055 - .060" thick. You could keep your factory stamp on the pad and still get a tight quench without decking the block or using a steel shim gasket.
Just an idea.
E85 racer and E85 carb builder
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68 Camaro, E85 powered 427" small block. 9.96 @ 133 MPH, 1.319 sixty foot on motor. 5.92 eighth @ 116 with a 1.42 sixty breaking beams with back tire on the bottle