Ok I was bored so as suggested by Rhiessan I went ahead and took the goal differential for the top 3 teams each year and the bottom 3 teams each year to get a better handle on the potential +/- advantages by playing in the 70s or the 80s.
As expected, even when selecting more of the teams, so that if there is an extreme outlier either way it has less effect, the top 3 teams of the 70s on average enjoyed a better goal differential of 72 compared to the top teams of the 80s.
The numbers speak for themselves. If you wanted to rack up some sweet +/- numbers, you wanted to play on one of the stronger teams in the 70s.
Season| Top 3 GF-GA| Bottom 3 GF-GA| Top3 - Bottom3
70| 188| -254| 442
71| 367| -287| 654
72| 353| -279| 632
73| 329| -393| 722
74| 345| -284| 629
75| 375| -513| 888
76| 378| -439| 817
77| 398| -239| 637
78| 415| -342| 757
79| 323| -263| 586
Average| 347.1| -329.3| 676.4
Season| Top 3 GF-GA| Bottom 3 GF-GA| Top3 - Bottom3
80| 278| -239| 517
81| 266| -327| 593
82| 394| -284| 678
83| 294| -387| 681
84| 295| -332| 627
85| 292| -296| 588
86| 253| -329| 582
87| 182| -135| 317
88| 227| -227| 454
89| 271| -216| 487
Average| 275.2| -277.2| 552.4
I selected the teams on their record (points %) to pick teams that had useful differences, and I think once or twice there was a tie that I did by wins then goals for but really it won't make much difference. Hopefully I didn't make too many mistakes, it's late!