Will 1965 Chevy truck 12 bolt rear end fit in a 1987 with 10 end?

No, a 1965 Chevy truck 12-bolt rear end will not fit directly into a 1987 Chevy truck with a 10-bolt rear end without significant modifications. The dimensions, mounting points, spring perches, and possibly even the axle width will be different. While some components might be adaptable, it's a major undertaking and not a simple bolt-in swap.