870 isx replacing a 871isx.

[Nostalgic]

You'll want to reuse the 871 wire harness, along with the intake manifold, exhaust manifold, egr cooler, etc. Just swapping the long block in basically. Also, you'll want to make note of the CPL of the 870 vs your old 871 and check quickserve to see if there's different injectors, injector cam and pistons to find if your old cal is compatible.