There are alot of mistakes happening assembling your engine. First install the main bearings in the block with assembly lube. Then install the main bearings in the main bearing caps with assembly lube. Look closely the caps should be marked 1-5 with an arrow to the front of the motor. To check clearances just cut a piece of plastic gauge the length of the width of the caps. Wipe the areas clean where plastic gauge is to be used. Torque the caps to specs, do not rotate crank. Loosen the caps and use the gauge paper to see what you have. This can be done one cap at a time. If everything is in spec torque all the main caps and spin crank over. Next lay out all the rods and pistons. Look at the big end of rods. Notice one side extends out more than the other side. These extended ends of the rods face each other when installed in the block. See picture for piston and rod assembly: Piston and Rod Assembly
. The rods and pistons should be marked for each cylinder. If not marked and have been mixed up, they will have to go to a machine shop.
They can put the rod and caps together and maybe match them. If not they will have to be resized. Once the pistons have been assembled to rod, install the pistons with the dot on top of piston facing front of engine. On the replacement piston measure from the center of the pin to top of piston and measure across the bottom of skirt to see if both pistons are the same. If you can, weigh each one too. Once the pistons are installed it will take a breaker bar and socket to turn crank over. If you do not have a press you will not be able to remove the pin from the piston and rod. Sometimes even at the machine shop the piston breaks when pressing the pin out.