Overheating. Scratching My Head.

Thanks for all the replies. I really appreciate your input. 

Easy to diagnose the water pump on the car.

1. Wait for the engine to be cold. (Let sit overnight or for 4 or 5 hours)
2. Remove radiator cap.
3. Start car.
4. Observe thru radiator fill neck the coolant level.
5. As enginge warms and t-stat opens, water will start to flow.
6. Reinstall cap or shut engine off.

If no water flow, t-stat not opening or pump impellar not moving.

To me it sounds like the t-stat is not installed correctly.

Air in upper hose can't prevent water flow through that hose. The water would push up and over the "hump" as quick as the pump could push it. Air in the engine is another matter as that isolates the thermostat, causes hot spots and so on. Thus the small hole in some states such as with the 4.0

But if you think of it - a whole lot of cars before the sealed systems, coolant recovery systems and so on had the "hump" with air in it because the radiator didn't run totally full meaning when you pulled the cap and looked - the upper hose would be half air because it sat above the level of coolant in the radiator. 
Air in the block, yeah, but the air in the hose won't block flow, it's not a solid or wall that could stop the pump from pushing coolant up and over and into the radiator. We'd have heaters not work and other issues otherwise. 

I never saw any response on the coolant type - AMC says that an engine running 205 degrees (can't recall their test RPM!) will pump 60 gpm with pure water - 73 gpm with a 50/50 mix - they were talking about over-heating due to incorrect coolant. 
As said, leaking head gaskets, that sort of thing, can cause over-heating in situations not normally seen. 

Geesh, I hope no one shoved the plugs into the block instead of removing them!

Odd thing - of all the cars I've worked on over the years, the AMC 6s - specifically the 4.0 I can think of, are the only engines I've seen where I had a problem with air trapped in the engine causing issues. Never had to do more than run a car with the CAP OFF and let them get warm, cool down, warm, cool down and they all solved themselves. Chevy, Ford, AMC, you name it. We did cooling system flushes, radiator work, thermostats, valve jobs, all of that on everything from Datsun, to Honda and all of the domestic makes - never any issue I can think of as far as having them not get the air out. 
Am I lucky or what?
Or do I just warm 'em up differently? Leaving the cap off, going through a couple heating and cooling cycles and so on?

Sorry about asking about the compression check as I see that you had done that. You mentioned that you took the t-stat out. Before pulling the engine apart start with the engine cold and the rad. cap off. Start the car up and observe the water flow in the top tank by looking through the rad.neck. Even at idle you should be able to see a noticeable flow of coolant
Something doesn't make sense that it was fine before you changed the hoses and now you have a problem. If the engine seems to be running properly and the timing is at or close to spec and you're not getting compression gases into the cooling system then there's hardly anything that could cause such a rapid heat rise other than impeded coolant flow. Either a water pump problem which is not that likely though it's possible that there's a problem with the impeller, like it being loose on the shaft. The impeller blade orientation should be the same as that of the pump in the picture you posted. I suppose you could have some severely mudded up blockage in the block but you'd have to pull the freeze plugs to check that.   

Boris Badanov wrote: Truth is that after all the good ideas I think that you have a junk radiator. I never had a AMC V8 over heat unless the radiator was junk.

That's my personal experience with all three of mine, and my brother's as well........ 
Eagle got hot only on the highway - radiator
70 Javelin got hot at lower speeds - radiator fixed that
73 got hot in town, not as bad on the highway - radiator fixed that
In fact now two of them run pretty cool - lower part of normal range. 
Radiators all "looked ok" and I even had one of them worked on by a shop.

Anyway, that's my personal experiences on MY cars.