I believe GM went to net horsepower numbers from the gross numbers previously used prior to 72. I do not have the data for the different years of 250's, but I would be fairly comfortable that if the compression ratios are the same, the true power levels are very close. I do know for the V8s, that the major change in performance was due reduced compression. 73 & 74 had the most restrictive emissions; in 75 things started to get better when converters and HEI were added.
