Puzzling engine failure. '71 AT1

So here's a puzzle that's been keeping my mind occupied all day.
I've got a 71 AT1 125. I put a 73 CT3 175 top end on it. From what I've read on the Internet, the bottom ends on both these engines are exactly the same.
So yesterday I was riding trying to get the jetting right and suddenly no power.
Then the engine and back wheel locked up.
So I pulled in the clutch and pulled over, thinking maybe it was just running too hot from being lean maybe. So I wait 15 or 20 minutes and the kick starter still wouldn't turn the engine over. So I put it in neutral and start coasting down hill (I was on a hill) and all of a suede there was a loud CLACK and the engine engages and it's turning over fine just like it wasn't just seized. But I was still in neutral and staring at the neutral light all perplexed and listening to the engine turning over and then it seized again, back wheel locks up too. So I pull the clutch and there was no response and I just skid to the side if the road. Couldn't kick it over, seized.
Couldn't roll it, wheel locked up; didn't matter that it was in neutral or if I held the clutch or anything.
So I called my dad so he could bring my truck and when he got to me like 45 minutes later, everything on my bike seemed fine: kick starter could turn engine, gears shifted fine clutch worked when in gear and engine was disengaged when in neutral.

To be honest, I'm pretty sick of working on this machine and my XJ needs a couple bills thrown at it, so the AT1 is being banished to the back of the garage for now... But my mind is boggling as to what caused this issue, anybody got any ideas?

Here's a pic before new top end:
http://i.imgur.com/1IiafOg.jpg

 

Seizure followd by con rod failure? The only way to know for sure is open it up. The root cause is probably having the jetting too lean, but if the autolube pump failed that'd to it too.

 

is it pre-mix or auto lube, im not familiar with your bike but on the rd250/350lc, they both share the same bottom end but the oil pump is different, some people dont up grade it when going from a 250 to 350 and it is under oiled a fair bit. as far as the rest of it goes did all the gears go back in as they should?if the nuetral light is on and its in gear maybe something came undone? as k moe said, you'll need to have a good look inside.

 

It's auto-lube. I put a pump for a 175 on it too. 1st 2nd and 3rd seemed normal after it cooled down. The clutch worked normally too.

 

I hope I'm not asking a stupid Q -
but when you turned your 125 into a 175 , what carb did you use ??
.
and if the crank seals go, that leans them out too.
so would old/bad gaskets.

 

I used the carb from the 125, bumped up the main from 130 to 230 then I think it has a 270 in there now. went from a 30 pilot to a 35. I replaced the crank seals this spring, and installed crank seal holders....

 

If the gearbox was locking up/doing weird things as well then I'd imagine it's got to be something worked loose internally. Of course the top end could have nipped up as well but that alone wouldn't result in a misbehaving gearbox, so as suggested I think the only solution is to split the cases & go through it with a fine tooth comb to see what's what :/
EDIT-: lol just noticed this is 2.5months old Did you ever get to the bottom of this, or was it one of those "throw it in the back of the garage & forget it for a couple of years" jobs?