Part # 70-1956 and 70-1956A are different lengths. 70-1956 has an under head length of 5.625" (5 5/8") and 70-1956A is 5.875" (5 7/8"). The 1956A is used for alloy 500 pre unit heads only. The head washers are .095" thick so keep this in mind when mocking it up.
Here are the under head lengths for the .375" head bolts I have in my notes:
70-1956 5.625" (w/standoff)
70-1956A 5.875" (w/standoff)
70-0327 3.400"
70-1484 5.625"
70-2873 5.750"
Hope this helps out with figuring out what you need- Dan