Just be aware that in 1981 Ford changed the balance from 28 oz to 50 oz, so things like harmonic balancer and flywheel or flexplate are different. Not a big deal, you just have to keep pre 81 stuff with a pre 81 engine and post 81 stuff with a post 81 engine. You can also buy special flexplates with the 50 oz balance to work with your C4, if that is the transmission you have. If you put the wrong balancer or flywheel on an engine it will shake like mad.

There may also be a difference in the drive belts..........I think 83 might be serpenine, but you can put your early timing cover and water pump on the later block and use a V belt system if you want.

Otherwise, it is almost a drop in.

Don