As far as ports and alloy go, the 848 head and the corresponding "850" 261 head are exactly the same. Only the deck height is differant (and the extra water passages are drilled) on the 850 head, as far as I can tell, to give 7.5:1 compression on a 261. Also, the 850 head has Stellite exhaust valves. The 848 may or may not. The valves and ports are the same size between the two, as are the rocker arms and the oil passages.
The 261 needs extra water passages to the head to prevent steam pockets, since it has siamesed cylinders. It also uses a bypass type thermostat on some applications, which helps prevent heat pockets while it is warming up.
The 848 head is put on 261's in an effort to increase compression slightly. If you are building a 261 from scratch it is not completely necessary since you can deck the block or mess with the pistons to increase compression. If you are putting a blower on the engine a lower compression head might be better.
