It's all about the blocks and their mounting holes. Pontiac used the same mounting locations and added extra holes in the later blocks to accommodate the different body styles that the motor might drop in to. The '71 block should have 5 drilled and tapped motor mount holes on each side. You'll only need to use 2.

Prior to 1970, the Firebird (F-body) and Tempest (A-body) body styles used the same motor mount holes. The Firebird was changed in '70, but the Tempest stayed the same, hence the multi-use requirements. Some later blocks - 1977, for example - don't always drop in to the Firebird without the proper adapters.