Of course, it's the 86 team. There were arguably the greatest NBA team of all time. Besides the starters, they had 3 bench players who were better than current Celtics bench: Bill Walton, Jerry Sichting and Scott Wedman. Horford, Pritchard, Hauser is not even close.
People forget how great the 86 team really was. They would also flourish in the new NBA rules. Bird would have been even more dominant in the modern NBA. He did what he did when hand checking and physical defense was the standard. McHale absolutely had 3 point range and would have used it much more in the modern NBA. Parish had range as well and his outlet passes were a major weapon. DJ was basically a better , smarter version of Marcus Smart.
Given enough time to adjust, I agree. 86 is one of the all-time great teams in the NBA, so the answer of 'the better team is going to win' is pretty boring.
This is part of the reason I might suggest that we evaluate this sort of hypothetical, inter-generational matchups as if you'd plucked both rosters out of thin air. You know how you see guys struggle to adjust when playing their first serious competition under FIBA rules?
It's all well and good to say 'McHale had 3-point range and could have been lethal from distance', because there's a small amount of evidence of that in his final season, but that
doesn't mean 1986 McHale is going to be hitting 7-8 at even a 30% clip if we somehow dropped both squads into a best of five or seven tomorrow. Bird is fine. Walton is fine. DJ is fine. Ainge is probably fine. Parish & McHale? They're going to have to adjust to very different game of basketball very quickly. I have no doubt they could do it over, say, a season. A series? Maybe, maybe not.
edit: the inverse is also true for the modern-day team playing under an 80's whistle, but the 86 team is simply better, so no reason to bring that up really.