Well, my 505 is a 4.350 (.100 over) and 1/4" stroke. I got my 4 bolt block for 100 bucks...dont plan on that happening too much, I sorta fell into the deal. It was available at my machinist's shop. He claims he goes up to .125 all the time on MK IV blocks...but they have to be sonic checked first to be sure. 1/4" stroke is really easy, but I doubt that 3/8" would be so easy, if it's even possible. I had to do a tiny bit of grinding at the bottom of some of the bores, just to be sure. The rod bolts came pretty close on some of them. Otherwise, it went together without a hitch. As far as building it yourself, it might be cheaper, it might not. And it depends on how much you like getting into it. I personally love doing it myself. Competition Products has a 496 assembled short block for $3595.00, not bad !! Im not sure of the max cubes on a std. deck short block. I would bet it's about what Im at, 505...not many machinists want to go to .125, but if you did, you would be at, Im not sure!! Maybe 512, 511 ?? Pat will know these figures, he does it all the time. John