Easiest way to get more displacement out of a 250 is to replace it with a 292 or 302. The 292 will be a lot cheaper, GMC 302 blocks are getting somewhat scarce and parts aren't cheap.
Other than that you can use the 292 crank in the 250, find the right rods and have a long stroke enginge with a lot of work invested. Or you can bore a 250 .120 over, but you lose block stability, which requires a lot of work to stiffen the lower end of the block.
