i usually mix up:
high pressure hose
kerosene and lots of it
a few tooth brushes
wire wheel on the angle grinder
die grinder
if youve got lots of time, it doesnt hurt to polish the tops of the heads and the valley.
a lot of old crud will form in the edges over the years, and the easiest way to chip it out is with the die grinder, so i usually get carried away and polish the whole thing. with fluffy, i didnt go to the extreme that i did my old 308 (pictured) but i did do all the oil returns and any way oil flows down back into the valley. this is called "de-burring" there will be casting dags all over the inside of the valley and you can really improve your oils return back to the sump by doing it.
couple of places to focus on for the ol 304 especially are the tops of the oil return holes in the heads. they often have sharp edges and are slightly raised, i MUNCH off the lip so its a nice smooth entry to the return hole. then underneath the heads hole in the valley, fully polish the oils path back to the valleys main return hole which i can almost guarantee will be blocked by 15-12% casting dag. ive done 3 308's and they were all the same in that area.
its kinda fun, if you have scrap oil, tip it over the area youre working on and watch it flow back to the sump, then clean up everything in its path.
another spot that is heavy on dags is the dizzys oil port in the valley. i was able to clean heaps of mess out of here so that oil flows from the valley to the dizzy easier.
clean up all around the bases of the valve springs and the channels the oil flows down to get to the return holes. just basically tip oil on a valve and watch which direction it takes, see if it pools somewhere and see if you can improve on it. keep doing it as long as you have time/interest. a lot of people will say its a waste of time because the difference in oil flow is negligable, BUT i actually
enjoy doing stuff like this, and it youre the same, then there is absolutely no harm in doing it.
to clean pistons i use a brass wire brush. its pain-staking, but you dont wanna scratch your pistons, so be careful. brake cleaner really helps loosen the carbon off the tops and sides. soaking for a while in kerosene also can help, but not as effective as brake cleaner. also remember the tops are not as important to get smooth and clean as the sides.
rods, pushrods, bolts etc all go in a container of kero and soaked for a few days, stirring it up every few hours, then they get wire wheeled. i stick my grinder/wheel in my vice then use the walls power switch to switch off on, the clean all the bolts etc on that.
exterior of the block gets done with the wire wheel. toothbrush/blade screwdriver in the areas the wire wheel cant reach.
once youve done that, everything gets high pressure washed/degreased 5, 6 maybe even 10 times, until you can wipe off every surface and no clack **** comes off on a clean rag. diswashing detergent helps to break down the leftover kero/muck at this stage. dont skimp on the washing. if a tiny bit of black crap comes off, wash the entire thing again.
i paint my block and heads at the same time, masking up the deck surface is a pain in the arse, so i find it easier to just hit them together. some people dont like the fact that the exposed head gasket gets painted, personally i dont give a ****.
to mask your spark plug holes, either use plugs your going to replace, or roll some masking tape into a cone and force it in the hole.
masking up areas like your rocker cover surface or water pump is really easy. layer on masking tape over the edges, press it all on firmly so the you can see the edges you want painted/not painted and use a hammer to gently tap the edges of the tape, for areas you cant reach, use a razor blade and drag it over the edges. this will perforate the tap and allow you to rip off the excess, leaving a perfectly masked area.
prior to painting wash off the block again thoroughly with prepsol (prepwas, trisol, wax-grease-tar remover, whatever you wanna call it) then either allow plenty of time to dry or hit it with heat gun/hairdryer.
hope thats been helpful. this is my FAVORITE part of the rebuild. sorry i get a little excited.