[AaronM]

1) There was a change in compression ratio as well as DME tuning. The later engines have a different computer running the EFI system and they have higher compression as well. That's why they have more power. If you used the engine from a 1983 car, you would also have to swap out the DME in your current car as well as the engine wiring harness.

2) While internal mechanical changes between the two engines undoubtedly mean there's some small weight difference, the weights listed in the chart you link are for the entire car, not just the engine. There were subtle changes to the body and interior for the 1987 cars and that's probably what accounts for the difference in total vehicle weight.

3) For the love of God, just take the car to a mechanic and have someone who's qualified do the work. If you didn't understand that the different compression ratios in that chart explained the 8 hp difference and if you honestly believed that the engine alone could weigh 2,637 pounds, you do NOT have the necessary mechanical knowledge to swap the engine on your own. This is not a simple "starter project" that you can just jump into and hope for the best.