400 blks. are o'k if they are your only options, i've got 2 of them and have had 1/2 doz. in the past. If they are prepared correctly they will handle 600hp for a time. The bottom end and cylinder walls being the weakest points. I personally drill out the steam hole and put pipe plugs in them (keeps them from cracking to the bolt holes)also fill the block to the bottom of the water pump bolts. I've cracked cylinders, broke main webs but have never blown one to pieces. I've had friends that blew the cranks out the bottom and split the steam holes to the bolt holes. This has never happened too me. The best thing about a 400 blk. is that they are getting harder and harder to find, most have gone by the wayside due to racing mishaps.
bottom line is the Dart block is Sooo much better and safer build. I got one of those too.
The 377" in my malibu is a stock block and welded, but i would not recommend doing what i did to keep it alive.
JMO
Zip.