If i get a piston thats bigger than stock, what do i have to do to make it fit??

 

You need to:

Have a machinist bore out the cylinder
Have a machinist hone the cylinder to the proper final dimension and to give it a good cross hatch pattern for good ring sealing
Make sure the ring end gap is properly set
Get a new head gasket that is made for the bigger bore

To get it all done right, take it to an experienced shop and give him the piston and rings as well as your cylinder.
Tell him to bore it out and hone for the larger piston and set the ring end gap. Make sure he gives the proper clearance b/w the piston and cylinder.

When you get it back it should be good to go.

 

Thanx for the tip