There is a link to an article that was written in the 60's somewhere on this site that is about building a 261. It reccomends milling the head but I'm not sure how much.

Over at Clifford Performance they reccomended milling the head 100 and recessing the valves.
I thought that recessing the valves you'll lose some or all of the gain from milling the head. So I milled the head 50 and decked the block 50 and then checked valve clearance by putting some modeling clay on the piston top below the valves and slowly cranking the engine, then I pulled the head and discovered there was no problem with clearance- Maybe you can mill the head 100 without recessing the valves?

Unfortunately I didn't measure the cc's in the cylinder and head to calculate my new compression ratio, and there is no other way to reliably calculate it.

'51 Chevy 1/2 ton w/'62 261, HEI, offy, fentons, dual carter/webbers, t-5 & 12 bolt posi