Actually...very.
The heads dont interchange as the front of the civvy head is narrower than the mil one...made up by the timing cover differences. The timing chains, rocker arms, cams and cam decks interchange. I am unsure whether the valves and seals interchange or not.
The cranks do not interchange as the mil one has a different pilot bearing hole diameter and different seal setups front and rear...the rods I am unsure...the pistons interchange though there are 7.5 and 8.5:1 compression flavors for civvy motors and mil ones are all 7.5:1. Oil pans do interchange as do the valve covers.
Intake, exhaust manifolds interchange as well as distributors, though the civvy one is 12 volt and has vacuum advance that the mil unit lacks. Civvy fuel pumps come with and without the vacuum pump...they do interchange with the mil one but if it doesnt have the vacuum part, you have to run the wipers another way. Civvy intakes come in 1 and 2 barrel varieties while mil ones are all the 1 barrel type.
Clutch, pressure plate, throwout bearing is the same.
Block, motor mounts, different.
Its a weird deal...2 relatively low production versions of the same engine with little in common.
Lord send your Holy Ghost into our hearts and make the desire of our hearts Your Will.
Pro-choice, that's a LIE, babies don't choose to die!!