Oowieeeeee........That’s the one that you’ve already had repaired correct
While round exhaust ports were all the rage back then,and would be a power bump over the square exhaust of the mark 4,everyone soon learned that the round bottom port floor was detrimental,if you look at chevys early L-88 round exhaust port versus the later "improved" D and C ports you can see their progression of thinking on exhaust port floor shape.But early on most of the head porters were on the round port bandwagon and there had to be a reason for that.At Super Chevy Sunday in the fall of 1983, the last one for Orange County International Raceway, and the day I got my '63 to run an 11.91, the Chevy Performance trailer had some displays, one of which contained an original Daytona Mystery Engine, and it was conspicuous in that it had large round exhaust ports. Definitely different than the later 396. That was probably only one of many unseen differences that added up to a big difference.
While those are some respectable numbers,not for a second do I believe that's all it has in it,the way those cars ran on the superspeedway I would expect them to be much closer to 500 hp,especially since the 421 SD and the Holman Moody 427 Fords were generally accepted to be putting out close to that.While that engine may be a smokey built engine it wasn't smokey tuned,and if they strictly followed the rules of the day then they were at an immediate disadvantage.Like all the good racers from that era,he may not have been able to recite what the rule book said but he could tell you chapter and verse what it didn't say.