Here is what I have and it's history as I know it.
My dad bought a 76 F150 2wd new. I grew up in this truck and it holds lots of memories for me. My dad drove it for a long time, then my sister's kids drove it for a while, but it eventually was retired to the back 40 and there it sat. About ten years ago I thought it would be great to re-do the truck and make it 4 wheel drive, as that is what I'm into.
I bought a 79 F100 custom 4X4 for $200. The body was very rusty, but I only wanted the drivetrain, which seemed to be in good shape. The VIN showed it originally had a 302 in it, but a previous owner had swapped in a 400. (The emissions sticker on the valve cover says it's a 77 model.) I stripped it down, and put the 76 cab and front clip on it. Life intervened and the project was going to the back burner for me, but my nephew was interested, so he took the truck. But, he did little with it and it wound up on the back 40 again.
This year I decided to re-visit this project, so I went and got the truck. When I last had the truck, the engine ran, but now the engine is locked up. When I got it back, the hood was detached, but sitting over the engine, but the air cleaner had been left off and the carb was exposed. I pulled the carb and there is evidence of water in the intake, but I don't know how bad.
From my research on this site, I've determined the 400 is from a car, because the oil dipstick is just in front of the passenger side head and goes into the block. Furthermore, since it came from a car, wouldn't it be likely that it has the "small" bellhousing bolt pattern? I'm thinking a likely senario is that only the engine was swapped, and this 400 was compatible with the existing C6 that was behind the 302.
Now my questions:
Is the 400 worth saving?? What can I expect to spend on a rebuild? (Say stock or mostly stock. nothing wild.) Say I spray some WD-40 into the spark plug holes and I get it to break loose and turn over, It would still need to be rebuilt?
Say I go the swap motor route, my nephew, (same one) has a 360 that I could buy. Would a 360 have the "small" bolt pattern? (As in the 302 pattern versus the 460 pattern.)

 

first off did you try prying on motor to get it to turn a little,or did it not turn at all,becaise if you did that would make it worse,put a lot of penetrading oil down intake and let it soak and put a lot in spark plug holes leave it for a week,get on the front of crank and try to turn it a little,might work but might bent pust rods,break rings,best bet is take intake and heads off and see how bad everything is rusted up, all you out is work and gaskets,

 

How did I manage to get this in the wrong section???
Admins please move this to the 335 section.

I only tried turning it over with the starter.
It did move slightly, but that's all.
Charged battery, new solenoid, new starter..

 

The 400 has a total different bellhousing pattern than the 302, 360 is different than both.