Great summary Antus,
you essentially answered all the questions spot on...
So now to the help.
to diagnose you will need to pull the motor down again. Remove all pistons and rods (dont drag the rod down/up the bore or across the journals on the crank.)
with the crank in the block and torqued down does the engine turn? if it doesnt then it is crank and block related. otherwise we now need to find out what has happened with the pistons. goto next step.
install two pistons (their pairs.) so not cylinder 1 and cylinder 2 you will want 1&4 and after those are checked pulled 2 & 3 go in. rotate after each assembly. You will need to verify the where the issue lies. I would assemble with molybond initially, this will keep lub up to the parts and allow you to diagnose. I would also coat the pistons and remove the rings. (removal of rings is a little dangerous as if they have run before they can be fragile. this is only to determine whether you have a journal issue piston issue or ring issue. If it locks up during the piston processure you will then need to find out which one. Do not assume there is only one. when you find the ones that are problematic remove the piston and bolt it down sideways with the block upside down, this is to rule out the piston and that it is the journal big end. if this is all good then look at the rings. fit and retest.
I suspect you will fail on step one, maybe step two. If failure on one then the crank might be bent or the block mains are not inline or the caps have been mismatched. undo one cap at a time and retest. if still locked then retorque that cap and try the next. This takes time. Now the fun thing.
MEASURING.
measuring you journals and bearings the right way means the use of calipers and verniers. take the crank out and install all bearings in the caps and block and torque. using the calipers find the size of the bearing and measure the caliper with the vernier. do this on each journal and measure in at least 4 spots. note all of it down. then use the vernier to measure the crank journals with the same measurements as what you did for the block. ideally the crank journals should be 3 thou smaller roughly than the block. if not the bearing will crab the crank. if you cannot find verniers or calipers there are two other options (I would normally use micrometers but these are specialist tools whihc you will nto just go buy for one build.)
go get yourself some plastigauge. This is not 100% accurate but will give a good guide. I have used this before with good success
engineering blue - it is like a paint, spray on the bearings lightly place the crank in torque up and then undo. Do not turn initially as you want to have transfer. where there is blue on the crank is potentially a tight spot. it sort of work in an absolute pinch. (must be done without lube, and the second time you go through this you need to turn the crank (if you can) slowly to show the tight spots, it should remove the blue off the bearing.
now if none of this can work pull the bearing and on the back it will display the size of it. usually as a 20 30 40 etc this is thou and for mains and big ends it is under size. If you have put 20 thou under but it is a virgin crank then you will have at least 17thou clamping your journal and thus the reason it does not turn. Verniers should be enough to tell you the journal size of the crank and big end journals which should be able to calculate the approximate gap. this will not find out an out of round journal or bore but it will give a indication as to whether you have the right bearings, crank and pistons. You might find a rod has been change and it is different to the rest. Generally the cap need to stay with the rods too, if you swap them this can cause an out of round, thus step two should find it.
More information to follow once you know roughly where the issue is.. above all good luck antus!
I know where you are at currently as I have help many others who are in this issue, but it should be possible to resolve. (if the category is IPRA then why not serious look at the 3.2 out of the rodeo... nice v6 running a delco with buckets of torque
cheers
oldn64
ps delcowizard beat me with a short answer, mine will help you find the issue. if it is rod related then we can try swapping them, but keep everything together to start with