"what do y'all think my compression ratio would be with flat-top pistons?"
If you have a specific piston in mind, the info that comes with the pistons usually tells you the projected compression ratio with different sized combustion chambers. If the data doesn't specify 58CC you can use any one of several compression ratio calculators available here on Hotrodders.com to find out.
They will typically give the comp.ratio for 64cc,76cc etc.
Here's the calculator I use...