Urban, I run a 505 on the street, 10/1 c/r, 291 heads which are basically the same as the 840's, solid roller cam with .655/.663 lift, 250/256 duration at .050. It runs really strong. I realize that we're talking less cubes with your combo, but that's offset somewhat by the fact that you're racing the car, and Im not. I also ran these same heads on a 461 (.030) a couple years ago, and it still had lots of low end torque. Im sure you give up a bit on the low end, but I doubt that it's even noticeable from a seat of the pants feel. Is this a race only car, or a weekend cruizer as well? It sounds like someone put together some good parts. Compression is a bit much for the street, what are you going to run for gas? That might be a problem. Otherwise, sounds pretty good. You would not see a difference going from your 330 runners down to 310, in my opinion. If you were to go to an oval port, now you're talking going to 265/275cc intakes, that's quite a difference. But then you would lose some top end power and rpms cause of the heads. I would stay with what is on it now. They're good heads for sure. If the car runs well as it is, why mess with it. If you can afford a set of good aftermarket heads next year for example, get them with a 310cc runner if that's what you want, and then you would also pick up the advantage of better flow characteristics, better exhaust port flow (BBC's were never known for their good exhaust flow!), etc.. Until then, I say let it be. Also, if this is a truck block as the seller is saying, all that means is that it might have come out of a pick up or suburban. The truck blocks that you are referring to are from big trucks or busses, they are the tall deck blocks. But they are a 366 or 427 c.i. only. They never made a 454 tall deck block. What they did do routinely is use a 4 bolt 454 in the trucks and pickups, and that's a plus. That's what I was lucky enough to find also. However, I dont really need it, and you dont either, a 2 bolt 454 is good to over 600HP without a problem, and lots of people have run them on a race only car for years and years with no issues. But it cant hurt!! I personally am not a fan of the tall deck blocks, only because you have to get a different intake, (or spacers), distributor, and the headers frequently dont fit right because of the added deck hgt. Too many different parts for me. But I realize they have a little more 'meat' to play with, so if that works for an individual, go for it. Good luck with the engine, and let us know what you decide. Pat is good at figuring out these kind of things, and me, well I just do what I can. John