I've been told on the last digit on the casting numbers is how many times that form, casting core? I can't think of the proper name for them now, was used on making that cast part I've seen some cast parts with -10 as the highest number on OEM Mopar stock castings.
As far as your heads can you do some measuring on the combustion chamber size ( outside diameter and CC) and the valve sizes as well as can you get a couple of light weight checking springs so you can set both sets of heads on your block to check and see if the valves hit the block or not when opening them by hand
I know nothing about the early Dodge hemi motors, I did run a 1955 New Yorker motor in my first hot rod and that got me into the early Chrysler Hemi motors and later into the 426 hemi motors
As far the change in which part Dodge did to make those motor bigger on the C.I. I would look up the stock bore and stroke sizes and go from their Think about LA motors with the 273, 318 and 340 having the same stroke and rod lengths same thing on the early Chrysler hemi, 331 and 354 C.I. same stroke, different bore sizes.
IHTHS