[vettestr]

Painting the block does a couple of things, it increases the ability for oil flow or return to the pan but it also seals the casting. These blocks are cast in sand so a bit of slag or crap can come loose and get in oil system. Block prep should include removing the slag and little corner globs of crap and seam flashing. Rounding sharp edges and radius of corners are also a benefit. This helps to reduce cracking from stress risers. Removing slag also means nothing to get into your engine and painting with glyptol also serves same purpose.
There are many folks a lot smarter than I and may think differently but some of these things if done wrong can hurt more than help. I am of the school of deburring all slag and rounding corners but not painting inside of block. I also open up some return holes and block others to control where oil returns to the pan. I do not want the top end oiling to drip on the spinning crank. I have already opened the rod side clearances a bit 0.003" for better cooling but this also increases splash issues.
I think for most home builders simply deburr and remove casting flash and stop there. Place screens on oil return areas to stop top end parts from getting to pan is cheap and easy with no down side. I also wrap oil pump pick-up with metal screen tied with wire in the tube to again keep crap out of oil. Use good parts and a great machinist and ENJOY your grenade. I say grenade because if you are pushing to point that much more than this is really needed then little parts from big parts are just part of life at 7000 + rpm. If it never sees more than 5 grand then this is overkill anyway.