Hello! After scanning the threads in the Nailhead forum plus some online searching I was unable to come up with clear ideas on how to make a 425cid nailhead engine into a 500+ HP monster. Think like you wanted a 12-13 sec 64-66 riviera, or 10-11 sec skylark from that era. Private correspondence with Tom Telesco seemed to indicate that one might be able to achieve 400+ (maybe 440) with the 425, but it would be expensive and time consuming. Perhaps a similar investment (time and money) would produce a 455cid with 550HP. From expert opinion (not mine - i'm no expert!) the biggest performance increase for the nailhead engine will come from extensive head modifications (porting, rockers, and valves), but this is also very expensive if done right. What about intakes, carbs, pistons, etc. What are the incremental performance improvements one can expect to see with nailhead modifications? If you want 500HP should you just punt the nailhead for other engine options (455, LS2). Thanks! Bert modification-------HP gain------------cost carb/intake --------20HP Etc. Etc.