Helo!
I have one SB 262 (bore = 3,67 in and stroke = 3,1 in.This engine is 110 hp originaly, and hte compression rate about 7,7 : 1.
My question is :

1- How I coul´d do in order to improve this compression rate to 8,5 - 9 : 1 ??? I had dificulty for find especials pistons for this engine....

2 - If I decide rebuild the engine to change it for 305 (cid) what I need to change ??

Thank you !!  

The 262 has an odd bore size and you will probably have a tough time finding high compression pistons without ordering them custom made.  If there is a good machine shop in your area you have an option to make your own custom combination. Take a 307 or 327 chevy crank that has a stroke of 3.25 and use it in your engine.  It will add .150 to your stroke.  The two things that will need to be addressed are the deck height and the balance.  
The bigger stroke will move your 262 pistons .075" higher than they were originally.  Most SBC engines come with the piston .025" down in the hole and if that is true then you will still need to machine .050" off of the top of the piston to keep it from hitting the cylinder head.
You will also need to balance the crankshaft.
The added stroke should increase your cubic inches to 275 and your compression ratio should go up to roughly 9:1 using your stock pistons  

Thank you for the answer!

In my calculation, I used the head volume that I found = 82 CC (head camera + piston camera piston in up position) - I did the practical avaluation, whith liquid, for this - , if I put another crankshaft - stroke 3,25, it will increase my compression for, at maximum, 8:1 .... where I´m wrong ?