An 848 head refers to the last three casting numbers found on the head just above the intake. There were various different heads made for the 235. The 848 head had a more desirable combustion chamber design, was made of a better alloy that is less prone to cracking and creates higher compression in they cylinders and thus more HP. It may also have had better valves from the factory but since you'll have new valves and valve seats installed to run unleaded gas, the factory valves might not be so important.
Here's my thoughts on building a 235/261:
1. Valves and seats to run unleaded gas.
2. High tension springs for a higher lift cam to avoid binding etc
3. 848 high compression head milled UP TO 100 to get higher compression (check valve clearance or recess the exhaust valves)
4. Deck the block for more compression (valve clearance is still an issue)
5. 4 BBL carb and 4bbl intake manifold or 2 2bbls and a dual carb manifold.
6. HEI ignition
7. Dual Exhaust with hedders or split cast manifold.
8. Drill the block for full pressure oil filtration and install a modern spin on oil filter using a remote filter mount.
9. 3/4 cam (Cliffords, Langdons, Howard's from Patrick...)
10. Drill the front bearing cap to install the two lower timing cover bolts from the outside so as not to have to pull the pan to change the timing gears or cam.
11. Drill and tap the crank and pulley to remove and replace easier.
12. PCV valve from draft tube to intake manifold
13. 4 core or aluminum radiator with fan shroud and auxiliary electric fan to keep that beast cool.
14. Chrome rings, they don't seat as quickly but stand up to overheating better.
15. Really overbore the engine (90 +) and then use v-8 pistons, see Langdon's Cadillac set up.
16. Balance pistons and connecting rods
17. Balance the crank (though may not be really necessary on an inline six depending on who you talk to)
18. Got money to burn? Fuel Injection
19. Wanna go fast, think about the transmission/rear differential, brakes, steering and suspension carefully.
