Install the pistons two at a time in opposite cylinders... Have the rod journal on the crank in the down position when installing the pistons...
Have you plastic gauged the bearings to make sure that the clearances are good???
As you install the pistons two at a time, you can see how hard the crank is to turn as you go... By the time you get to 6 and then 8 pistons, you should need to have a wrench on the crank to turn it... If it is too tight to turn, then back off the rod caps and loosen the caps two at a time and then try to turn the crank... When you get to the ones where the crank is easier to turn, then look at those two rods and caps carefully...
PS: Make sure that you have the bearings clean and the bores in the rod that the bearings go in absolutely clean... I've seen a small spec of dirt or piece of fuzz get on a bearing or behind one and make it tight and hard to turn... If that happens, you should see a shiny spot on the bearing surface that contacts the crank if that is what caused it to be tight and you were able to turn it with a wrench...
Clean both sides of the rod bore for the bearing with a tack cloth before installing the bearings, and then tack cloth the bearings and then the crank bearing journals on the crank before installing the pistons... Make sure that your hands are clean and free of dirt, even if you have to keep washing them after each set of pistons...
Make sure that the rings are not overlapped also....
Then make sure the the bearings don't get knocked out of whack and overlap when you are installing them - it's happened to me before... make sure that the bearings are properly seated in the bores and that they butt up to each other properly when the cap mates to the rod...