Hey,
Does anyone know what I would have to change to switch a'78-'81 Malibu 4-door from a six cylinder to an eight?

 

Is this a Chevy V6?

If so, the trans will bolt up. You'll need a motor, exhaust, and a bunch of accessory brackets and such. You can get suitable stuff from almost any full-size or mid-size car from the early 80s with a Chevy V8, like a Caprice or a V8 Malibu. You might need motor mounts, the frame side that is, I don't recall if they're different or not in those cars. One of my little brothers did that swap in about a 78 Monte Carlo, didn't have too much trouble with it IIRC. One of the easier motor swaps.

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"So many Mustangs, so little time..."
ICON Motorsports

 

You will need mid-size car motor mount brackets and Malibu motor mounts on the frame. Truck and "big car" mount backets are
taller and the bolt spread will be too wide to bolt the upper and lower motor mounts
together. Use a V-8 harmonic balancer with the V-6 brackets and pulleys. This advice is
given with the assumption you have a Chevy
V-8 going into a Malibu with a 229 V-6. You
can also bolt the 229 Y-pipe to the V-8
Malibu exhaust manifolds if you don't want to
use headers.