Sounds like you already buy race gas by the drum. What's one more? Me, I hate to admit I've made a serious uncorrectable blunder. Now, with that said, it doesn't sound like you have the skills or the inclination to fix this problem yourself. If you did, you would pull it apart on the stand where it sits and see what you got. Most folks I know would never buy a used race engine from anything less than a reputable engine builder. So...that leaves you with the option of breaking it down and selling for salvage the parts you don't use. Use the block, the rods and crank and buy new parts that fit what you intend to do with the car. 500 HP on pump gas is not difficult with a 355 if you use a set of AFR or comprable heads and a hydraulic roller cam. It's at the upper range of what you can expect without power adders.
Maybe the guy you bought it from will buy it back at a discount you can live with.