87 to 79 engine tranny swap.

I understand an 87 2.6 will not bolt up to my 79 automatic transmission.
Would I be able to swap in the 2.6 and auto tranny from the 87 into my 79?

Short answer - yes. It may not be a straight up swap as there might be a difference with the transmission mount and front tail shaft yoke but as both trucks are auto the trans tunnels won't cause you grief. So what's the story with your Gen 1 truck engine?

I picked up a D50 motorhome with seized engine a couple years ago.
Was going to swap in a 318 with an A904 tranny, and ran into a problem with rebuilding the 318.
So decided this might be the route to go, the 87 came up cheap and is only 3hr drive to go get it.