Has anybody used a 1976 thru 1978 440 block for racing and are the RV Blocks a good block to use. These blocks are supposed to have thicker main webs.

The later blocks tend to have thicker cylinder walls than early blocks, but the material is a little softer. I think the HP limit is the same for all production blocks regardless of the casting number. Once you're over 600 hp the block will fail. Sometimes quickly, sometimes it takes a few seasons.

Some of the later blocks have a little thicker main web and others do not. You have to measure each individual block. The RV blocks are no stronger than the passenger blocks. Yes they can be used for racing.

Andy, have you used any of these blocks, I am thinking of using one with hardblok just because the main web is thicker.Looking at building the basic 500 CI for drag racing around 750 hp.

start out with good cylinder wall thickness, aluminum main caps, hard fill it, keep the rotating weight reasonable along with the rpm, keep it out of detonation, deburr the block and remove all stress risers. (there are plenty)
those are the things that help keep those engines living in that HP range. there's never any guarantee, but if you don't do most or all of that stuff i can pretty much guarantee it will break at 750 hp.
regarding breaking at 600 HP, i'm not sure sure about that. i've built dozens of street/strip 600 hp motors over the last few years making more than 600 HP and not one has cracked that i'm aware of. 600 is easy to make with a 500+ inch combo.

well looking at them i can not imagine they are not stronger. i have two of them. we are gurdling one for my duster and will hardblock it half way>

Yes, I've used the late model blocks. The hardblock will help some but you'll still break it eventually at 750 hp. It might last a few years before it goes. You'll just have to decide if that works for you or not.

If you catch it early enough then you can just replace the block and keep racing. But there have been lots of guys on here who lose the entire motor when the block breaks. That gets to be expensive and can also be dangerous if it breaks while at the track.