Engine Seized Motor?

[Zenki 5.0]

Hello! I am somewhat new to engines and working on them, but I'm not completely dumb.

A few months ago i bought a running 5.0 302 to swap into my 1995 240sx. The 5.0 was already in a 240sx, just an earlier model. This was a turn-key swap, minus cleaning up the bad wiring and a driveshaft that needs to get lengthened. Everything mounted right up (minus banging the tranny wall in for clearance). Some time and a lot of wiring later, I get it to run again! Only a minute or so each time. Had struggles, but overcame them.

NOW, I started the car up just like normal, but it died a little after, like a minute or so. Didn't hear anything weird, nothing out of the normal. Plugged a vac leak, went to crank, nothing. Battery is fine. Manually cranked with breaker bar, wouldn't turn in either direction.

So, is it safe to say the motor just happened to poop on me? Didn't make any strange noise when it died.

Here is what I have tried while manually cranking it over:
-no d shaft in (in/out of gear shouldn't matter)
-pulled plugs
-took off serp belt (accessories are not seized anyways)
-starter is out, teeth not engaged
-checked oil, just oil! (No fuel/coolant)
-borrowed a borescope, all cylinders seem to be fine, same as pistons.

Has anyone experienced this? I have looked at many threads but haven't found anything. Not hydrolocked, something else has to be it.

If anyone can link me to an appropriate post, or lay out what you know, please do so. Again, I'm kind of new to working on cars, so I may have over looked some things. I appreciate you taking time to read this and thanks in advance to those who can possibly shed some light.

Thanks again!

 

[General karthief]

What trans are you running?
Did you put engine and trans together or where they linked already
Did you here it run in the other car
When you started it up did it sound like it was laboring, like it was under a load or didn't want to rev up

 

[Zenki 5.0]

What trans are you running?
Did you put engine and trans together or where they linked already
Did you here it run in the other car
When you started it up did it sound like it was laboring, like it was under a load or didn't want to rev up

T5 trans. The guy started it up so I could hear it. It screamed haha, no stress there. I bought a diff tranny. Put it all in as one. Once I finished wiring with the motor/trans in my chassis, it was having a hard time revving, but idle'd fine till I shut it off. Now, nothing at all.

 

[RaggedGT]

Possible flywheel problem?

 

[Zenki 5.0]

Could be. Don't want to rule out the engine as not the problem, but the trans could def be the problem. Need to get back under the car or take the thing out to get a better idea.