My question is: what are the effects of removing the crank before the heads? Block flexing / torsion?
i'm thinking that the direct effects may be more along the lines of a piston sliding up the bore and dinging an open valve or scoring the top of a piston. the attached connecting rod scratching the cylinder walling during a slide. i don't know if a piston can move up a cylinder far enough to damage a ring on a ridge(if ridge has developed) with the head on, but something will hit something IF a piston moves that way uncontrolled. maybe six conrods falling as they clear their journals going dink, dink, dink, dink, ding, dink, dink as they contact the bottom of the cylinder wall might start some kind of injury.
i have a question, and please bear with me as i may be misunderstanding your intent. is it really necessary to remove the crank to clean out any debris from the chain guides? the only oil passages the crank blocks are the oil passages under pressure which means the oil just got screened at the oil pump pickup, pumped through the oil filter and should be (somewhat) clean. the t.c. tensioner and the guide are, as far as i know, in an area where oil that has left the lube system is dropping back to the oil pan(i'm not intimately familiar with this engine and am using knowledge gained from working on many other chain driven valve timing configurations, so if i am incorrect will someone please inform me). it may be necessary to just remove the oil pan to clean the screen and nail the inside of the pan with a can or two of brakecleen. that, and the new filter you'll install should get rid of most of the bits and pieces of concern.
also, if only the crank is removed the pistons would still block the view of the valves.
although not asked i'll make a suggestion regarding head removal from my lazy guy perspective: unless you insist on removing the crank, just change the chains, guides, and tensioners. remove the pan to clean the screen and make sure there aren't any big chunks in there that won't fit through the oil drain hole. install the engine and before you bolt on the exhaust manifolds slap a compression gauge on the cylinders to see if there may be some indication of valve problems.
you can check the exhaust valves for leakage before the engine is installed by rotating the engine on the stand so the exhaust ports are facing straight up and pouring some solvent in the port(s) that have a closed valve. any leakage would present itself rather quickly.
alright, i'll be honest with you, i'd just change the parts, change the rear main seal, clean the pan and screen and bolt everything back together. if the truck runs relatively smooth and strong the job is considered done and i'm down the road.
but you seem to be a bit more meticulous.