I don't know about the LT1, but the LS1 has 6-bolt mains so they are definitely a stong block. I don't know if they'll handle 1000HP safely, but I wouldn't be surprised if it was close to that.
idk if you guys have seen it, but there was a vid on svtperformance.com a while back of a crazy supra's dyno pull.....it was REDICULOUS....it pulled 1100 hp on the stock block, and over 800 ft lbs......its wierd cause the powerband on the supras is like no other car.....its got like no tq up to about 5500-6000 rpms, but at about that rpm, the tires just get hammered....in one of the pulls, one of the straps ripped from the tq.......
Supras can handle close to 1000HP on the stock block and they have a decent amount of torque as well somewhere in the 9xx's. It's probably the only Japanese Car I would ever think of owning.
03 Cobra can handle a butt load of power on the stock block also.
The Viper's can also handle over 1000 HP on the stock block.