I have a 1984 Ford F-150 that is a short wheel base with a stepside bed. This truck has an automatic transmission (C6 I think)
The aging 302 doesn't sound like its going to last much longer.

I have 1981 Ford F-100 that has a good 300 6 cylinder in it. This has a 4 speed standard in it.
Other than the flywheel, Will the 300 6 cylinder bolt up to the automatic transmission and sit in place in the 1984 Ford F-150?
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Yes it will bolt in place to the auto tranny. However the motor mounts and the engine perches are different between the two. The 300 302 and 351W all have the same bolt pattern.

 

Since you have the other truck, you will have everything you need to get it going. All the little stuff like throttle cables, brackets, etc.

You may have a problem with the radiator. You have a six cylinder radiator you could swap in, but it doesn't have the cooler fittings for the auto tranny. So you might have to buy a radiator.

 

Or, you could buy an aftermarket tranny cooler instead of a radiator.