Will Chevy s-10 rear end gears fit a 1985 montecarlo?

No, a Chevy S-10 rear end will not fit a 1985 Monte Carlo without significant modifications, and it's likely not feasible. The S-10 uses a much smaller, narrower rear axle housing designed for a smaller vehicle. The Monte Carlo's rear end is considerably larger and wider. Even if you could somehow adapt the housing, the mounting points, spring perches, and overall dimensions would be drastically different, requiring extensive fabrication and likely wouldn't be a worthwhile endeavor.