Sounds like you have a fun toy!
Retired racing engines are always a good find. Dub has them on the Hangar Sale requently, and lots of folks are thrilled to get an engine like that (as Dave noted, it lost 300-500 rpm for racing, but it still screams compaired to most)
Of note....
The Jett SJ-50 is rpm limited by the stock jett-stream sport muffler (15,000 to 18,500 rpm range). This set up intended for sport props in the full blade 9x8, 9x9, 10x6 and 11x5 range. It will actually turn the same rpm as the Q-500 engine on the 8.75x9W prop, by using the Q-500 muffler (18,500 to 21,500 range). Dub sells SJ-50 engines configured that way for folks sport flying Q-500 birds quite often. The limitation is you now must use the q-500 props to keep the engine up in that rpm range. The advantage here is with the sport engine port timing, the engine still has a perfect midrange and idles great. And you can always swap back to the longer muffler and use it for sport flying.
Dub also offers the 428 Q-500 engine with the red carb installed as an option, which is about what you have there with the perry/Nelson comination. It only loses about 300-400 rpm off from the racing 'air valve' carb. And, it will idle and transition - just does not like to run in the midrange for a great deal of time, and you are restricted to the Q-500 type props to really allow it to run well (will not run worth a hoot with the longer muffler).
(edit - boy my spelling just great today........shhessh!)