spark plugs welded in, bottom end still good?!!!

Doubt it...generally when engines "blow up" its the bottom end that grenades...thus the term. Now you may be able to save the crank...MAYBE, but I wouldnt look to keep anything else.

If it got THAT hot, the pistons are most likely fried...and if you had any impacts, your rods could be bent or broke. I wouldnt trust them though after they were put under any sort of stress. A stock shortblock is dirt cheap anyways....so not a huge deal.

What exactly happened? The only way I can see the plugs getting THAT hot is from lack of oil pressure...or were you spraying when it exploded? A lean condition at that point has been known to actually melt pistons and melt the electrode off plugs....so obviously the plug would get darn hot too...possibly welding itself in a bit like you said.

So were you just driving around, going WOT or going WOT W/ spray when it let go?

You'll have to pull the heads and even then you can't be certain.

Pull the heads and look for scoring on the cylinder walls. If they are scored, the engine overheated badly enough to do damage to the bottom end and it will need rebuilt or replaced. When you overheat, either to pistons will grow and deposit aluminum on the cylinder walls or the ring ends will touch and cause interference.

You'll also have to inspect the top of the pistons. If they are melted or distorted, then you'll have to take things apart.

Still, checking these will not tell you for sure if the bottom end is ok or not. Your best bet is to tear the bottom end down and check all the clearances.

I can't imagine you did this damage without some very bad detonation. Did you have the car tuned after putting in the 10:1 pistons with PI heads? That kind of compression is the very upper limit of what you can do on premium pump gas, and you'd still have to back off the timing to prevent detonation.