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If you can, get a Buick 455 that matches the year your car was made. I am not sure if its possible to tell with Buick, from the serial numbers. I know it is with Pontiacs and generally not with Olds.
The power decline you talk about was largely on paper.
Up to 1970, the power ratings were SAE 'gross', where all the power robbing accessories such as exhaust, water pumps air cleaners were not installed. There was also a LOT of advertising and fiction involved in their publication.
In 1971, GM went to SAE 'net' where the power was measured as installed in the car, with all equipment attached. This process is much more accurate.
Therefore, the 370 hp 1970 455 is not accurate and is likely almost identical to the 250 hp 1971 455.
There were 2 differences, though. 1971 and later had hardened valve seats, suitable for unleaded gas. Earlier engines will last longer if you use a lead suppliment.
Also, compression ratios dropped in 1971. You lost about 4% of power for every 1 point in compression dropped, but it will run better on todays gas.
Many high compression engines from the '60's will rattle like 6 lug nuts in a coffee can even on our premium unleaded today.
Finally, even a lower power 455 can really be boosted by some simple bolt on parts and careful tuning.
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