Valve Clearance
 

The only way to know for sure is to measure PTV clearance with the cam you intend to use before milling. It's easy to shave the heads....much harder to put it back on if you guess and get it wrong.

PS The LY6 came from the factory with LS3 style heads, 821,823, If you know your heads haven't been milled before you got them then you might be able to find out what other LY6 owners have found clearance wise. If they have been milled then you'd just be guessing on how much further you can mill them.

Do you have the heads, cam and a adjustable pushrod checker?

If it's not worth doing right then why bother? How much time and money do you think you'll waste if you get it wrong? You could get install the cam and trying using the stock pushrods to check PTV clearance. If that doesn't work get a pair of the correct length pushrods or even buy a second adjustable pushrod (you should already have one right?)

I used my adjustable push rod checker, set it to the correct length. You need to know your pushrod length for checking ptv

I already knew what pushrods I needed but had not ordered them yet. I had my heads/cam/ head gasket and it was just a matter of if I needed to cut valve reliefs or not.

You're going to need a pushrod length checker to get the pushrod length set at some point so why not get a couple now, You could always sell one or both when you're done with them.

LY6 should have had 823 "type" rectangle port heads on it to begin with (maybe 5364's, or....). Also LY6 is a VVT motor, so I'm guessing you should have (intake) valve reliefs already cut into the stock pistons? I'm only speaking from my L92 experience, but same engine family. Does the cam you want specify 10.5:1 target compression ratio? There should be a ton of info available for compression points gained per amount of milling. You can tweak head gasket thickness as well, but keep quench in mind if you want the best out of it.

For me, I'd put the cam in you want, assemble one head side with a light valve spring for checking, and spin the engine over with a dial indicator on the valve, and hand push the valve open further each few degrees to check and measure clearance. Then you would know your allowances. Maybe a dinosaur method, but should work. Also ask the cam maker, they should have knowledge and experience of guidance level advice, but it always ends up as final check on your end.

Stock bottom end with a zero deck and 0.045 compressed thickness gasket you would need to mill the heads to a 63cc combustion chamber in order to reach 10.5:1.