dishing 255 pistons

So I just got a kick in my butt from the body shop about my mustang and the short of it is that my engine build budget just got shortened by $2k.
Now the rocker arms and pistons I was planning on getting are gonna go to the back. Now here is a question for you guys.
How much do shops charge to mod stock pistons? or if I took the pistons and used the dimensions on a 13cc dish and cut them out, would that affect the piston
negatively?
I have a guy that is in my VFW that offered to let me use his milling machines as long as I am cutting aluminum.
What do you guys think?

Keep in mind that pistons are made with the crown thickness at a specific thickness whether they are flat top, dish or domed. So taking a flat top piston and machining a dish in it may make the thickness inside the dish area too thin. So yes, it can have a negative affect if it ends up too thin after you cut a dish in it. They really aren't made to have more than just a minimum of material removed from the crown.

This is the information I was looking for. The only reason I was thinking of doing this was after reading something on fly cutting them for valve clearance.

Here is my next question. If that is not a very good idea. What would be the best option piston considering I am going to need to use stock replacements?
An what is a safe CR to run with them? I was planing 10:1 with my AL head.

Yeah, aluminum heads are more forgiving when it comes to compression. Something to consider is you can always adjust your DCR to compensate for too much compression. You can have over 10:1 static compression, but with the right cam your DCR could drop below 8-1/2:1 or lower, since that is what the engine recognizes anyway, the static compression is just incidental at that point. As long as you can get your DCR into a safe "pump gas" zone, stock replacement pistons will be fine.

I have been trying to keep the DCR math in the 7.9-8.1 range. I have read different things and those numbers seem to be safe with 91 octane from what I have read.
Is this correct? or would you go a little lower?

Yes, I would agree with those numbers being safe with pump gas. Advancing or retarding the cam also changes the DCR as well, so you can play around with unlimited cam selection scenarios and calculate the advance/retard possibilities until you find a combo you can have as a good compromise.

Well those numbers are with 4* advance. I have read that the clay smith cams preform good with that advance.