I am looking at the ratings of the stock 250cid over the years, trying to figure what changed.
250/155hp, 1966-1970

250/145hp 1971 -
what changed? Smaller carb? Other?

250/110hp 1972
I know this is a NET rating instead of GROSS, but any other changes vs 1971?

250/100hp 1973-74
Loss of 10hp at least in the rating.
Changes? Retarded cam? Other?

I don't have enough manuals from this era to figure it out, thought somebody might know what changed in these years.

The meausrement changed from the engine @ the crankshaft,or the back of the transmission ,maybe engine w/no accessories or something like that?

I really do not think the engines were any different 66-72.

At least I do not think there are any changes.

MBHD

AIR pump - run rich to reduce combustion temps and NOx products, then inject fresh air into the exhaust to complete the burn

EGR - pass inert exhaust gas back thru the intake to lower combustion temps, again to reduce NOx products

Both of these emissions techniques were being employed about this vintage. I don't know if this is the answer, just suggesting it may be related. I do know I have seen early '70-ish intake manifolds with EGR hardware. I have seen AIR pumps too about this vintage but don't recall if it was on an inline.