The "848" head was first used in 1956. It was the replacement head for 53-63 235 engines in the 1957 and later MPBs.
The NAPA reducer had to be reamed but I do not think that the Classic parts one has to be reamed. I'll try to remember to check the other NAPA part and the Eaton/Weatherhead part (next time I am at the main NAPA warehouse).
I think that Pre '68 Dave once explained why/how the compression changed on later 235s (all with the 848 head)? Carburetor might have been the reason?
Here is a good engine-specs website. 
