What all do I need to do to take a 6.9 international out of my 1985 f250 and replace it with a 7.3 from a 1986 f250? The only reason I am doing this is because my 6.9 has 2 dead cylinders and my buddy has a good 7.3 that hes willing to sell me.

 

I think they still had the 6.9 in 86. At least that's whats in mine. Can't help you on the rest but if it is IDI it should be the same as far as I know.

 

Yep, the 6.9 was used through '87. If that '86 has a 7.3 it's not original.
Pretty straightforward swap though, externally the engines are the same.

 

Unless its a newer motor it should just slip in

 

It wouldn't even matter if it was newer up to a 1994 engine. The only one that wouldn't work is a powerstroke. And they will, but not without a lot of extra fabbing and work. Any 6.9 or 7.3 IDI will fit right in there and work like it was factory.

 

No the wire harness is different after 92 cause I did a swap from a 92 into a 91 and I had to make the harness fit

 

Harness and engine are two completely different things for what OP was asking. The only thing he'd need to change on a newer engine would be the glow plugs, or glow plug connectors.