When you say seizes:
-Is it rpm related or throttle position related? See, you can go wot at any rpm, right? And are you saying the engine goes from 5000rpm to zero rpm in like 1 second or 5 or what? And if its rpm related, can you get it to seize at part throttle? Does the engine bog or hesitate first or just flat quit, like you shut it off? Is there any sign of missfire or backfire?
-When you say it wont restart; does that mean:A) it wont crank? Or B) cranks but wont fire? If A) it wont crank, have you tried to manually turn the engine over? If yes you tried but it wouldnt move, then remove all sparkplugs and retry. If it still wont move, well then get the rear wheels off the ground, put the gear selector into neutral and retry.If it still wont turn over, then it really is seized.
-Assuming it "really is seized",you said the oil pressure is "good", so lets assume the bearings are all good. Inside the engine the only thing left, powerful enough to stall the motor at wot, is the pistons and rings. Since you said it runs fine and didnt mention burns oil, lets also assume the rings are "good". Well that just leaves pistons. However after several seizures, there would have been enough metal transfer to tear up the rings and you would have a smoking engine, which you again didnt mention. So, conclusion is; its not the engine.
-Moving along; Some have indicated a possible bad convertor. Well lets think about that.You said it "runs and idles and drives fine".And it happens in park also. Well if the parts inside the convertor were to seize, I cant imagine that you wouldnt have mentioned how crappy the acceleration was, or something about it stalling when you put it into gear.So lets move on.Lets go to the hub and pump.If the hub were to seize in the front bushing, it would just tear it out.If the pump were to seize it wouldnt stall the motor at 5000rpm. So moving along;The only other thing indirectly connected to the the crankshaft is the input shaft to the rear drum(forward clutch). Lets see, how could that seize at 5000rpm and stall the motor? Im not getting a picture.....
-Heres what I am picturing.Going back to it only happens at WOT.Again, assuming, your KD linkage is functioning correctly, lets think about whats happening. Your cruising along in Drive and you mash it. The KD linkage calls for a downshift. Depending on your roadspeed, the trans will want to go to 2nd or 1st, right? But its still in Drive. What if the trans is finding two gears at once? .......Instant stall.How can this happen? I dont know, but we will have to prove it.Or disprove it. I would be tempted to disconnect the KD linkage and try it.This will prevent a WOT downshift command. If the problem goes away, now youve learned something.If it persists, the theory could still be right; it just not KD related. It could be a ValveBody issue.
-Now on to the restart.If the engine stalled due to a tranny locked up, at wot, chances are very good that it is flooded.And, as soon as the pressures in the tranny leaked down enough to free the clutches( a few seconds), it should at least crank.Unless it hydrolocked.
-Now getting back to tranny locked-up.In a mopar tranny there are 2 bands and 2 clutches. On a stock VB when any forward gear is selected,the forward(called rear clutch) clutch is engaged.For manual low the rear band is energized as well.For 2nd the rear band is released and the front band is energized, as well. For Drive the front band is released and the Hi-drum (called front) clutch is engaged,as well. When reverse is selected,the forward clutch is released, the rear band is applied and the hi-drum is engaged. Its easy for me to imagine a failure in the hydraulic circuits causing a two-gear lock-up.Esp the 3-2 downshift circuit, which is hi not releasing while front band is applying.Or 3-1 downshift, which is hi not releasing while rear band is applying.
-Ok my head hurts....