You're right about 70 vs. 71. "Net" hp ratings didn't come into play until 1972 (what I thought...)
1975 Motor's Auto Repair Manual
1970 - 250 hp - 345 ft/lbs - 9:1 compression
1971 - 245 hp - 350 ft/lbs - 8.5:1
(the 5 ft/lbs probably made those two feel the same)
1972 - 165 hp - 280 ft/lbs - 8.5:1
The 71 and 72 specs are the same across the board - dwell angle, plugs/gap, and timing. Same power, difference is "gross" and "net" ratings. GROSS ratings (prior to 1972) are measured using no accessories/air filter/exhaust. NET ratings REQUIRE as installed accessories (base model, AC compressors and PS weren't on there, just water pump and alternator -- unless all models had the other items as many do now), air filter and housing, exhaust manifolds, and simulated factory exhaust system.
In reality there was no power drop between 71 and 72 models, just the way they were rated changed.