The 1956 engine wasn't designed for shims. There should be dowel pins only in the rear cap and caps 2 and 3 should be marked F and R (sometimes you have to look for these) showing which goes forward and which faces to the rear. Let us know if you need more info on this...
Jon


~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end