Piston Question

That is a good thing to worry about. Buuuut... I guess you mean 60779 heads and a Felpro 1008 gasket? (That gasket has a .039" installed height BTW.) We have those very same heads on our 340 and the pistons are out of the hole about .004", and have valve reliefs. We had GOBS of piston to valve clearance with a moderate cam and 1.6 ratio rockers: around 268 advertised duration and .490/.505 lift.

Your pistons are .100" in the hole and would be .080" down the hole with the taller compression height 526's. So it is only the lack of valve reliefs that you need to be concerned over, AND your particular cam. It is easy to find out: mock up the assembly with the cam and valvetrain and piston/rod for 1 cylinder and turn it over with some clay on the top of the present piston and see how deep an impression it makes relative to the flat top area. Then you can figure out if .020" extra height and no valve relief will be any issue.

The cam is what you have to look out for. BTW, I re-ran the SCR numbers with your parts (assuming 63 cc's on the head chamber), and got 7.51 with the 285 pistons and 8.55 with the 526 pistons. That 1+ point in SCR will be worth a good low RPM torque boost in street use.

What cam are you running? And what kind of running will you be doing? Cruise? Pure drag race??

And who is 'they'? The machine shop?