http://forums.luvtruck.com/viewtopic.php?t=2083
Outlines the year changes to these trucks. The most significant, is the engine design change to a crossflow head.
From what you've described, the extra engine you've got is of no use to repair your '79. It's an early model G180.
A lot of the guys here are very good mechanics, I'm not so much. With that in mind, in your situation, here's what I'd do...
Go buy a $10 bottle of 'liquid head gasket' at Autozoned. This stuff is a patch, but it might help you determine what is going on with your engine. Drain the coolant, and backflush with water. Put the liquid gasket in, fill the truck up with water and see what happens when you start it. You may want to loop the heater core water pipes together to keep this stuff out of the heater core. If it's leaking water (I had a freeze plug rot out on my '78 ) then this won't fix anything. If it's pouring white smoke out the tailpipe, this just might solve the problem temporarily.
I know I'll probably get flamed for this, but I'm a cheap kind of guy and if the problem exceeds my abilities, I like to know before I invest too much time or money.
If the truck was overheated due to a bad headgasket, you will want to check the block and head for warping once you take it apart. Heads do come up on eBay from time to time, and there might even be one available in the classifieds here on the site. If you've got the skill, there are plenty of folks here who will help you port match and rebuild your head.
Good luck, let us know how it goes...