There will be a slight loss of compression if you're replacing an 848 head with the 570. No, there's no need to drill the 235 block for the holes in the 261 head- - - -they would serve no purpose since the 235 is able to flow coolant between the cylinders. Use whatever gasket is handy. Without the corresponding holes in the head and block, there's no reason to worry about whether or not the gasket has holes in it. I'd suggest using the 235 rocker arm assembly to be sure the rocker oiling system stays the same.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!