Stall converter recommendations

i plan on using this car for mainly racing and summer weekends. I just had my 360 built and i haven't got it to run right. I was shooting for 400 horse but i feels more like 200. Im running a 360 with 10 to 1 compression, oregon cam grind 474 lift and 289 duration, 2.02 1.60 ported and polished heads, edelbrock air gap intake, 750 carter avs, and 340 manifolds. For the tranny its a 727 with a stage 2 shift kit, and a factory 340 high stall, and then a 8 3/4 with 323 suregrip. I dont have as much power as i expected. So i just picked up a set of headman headers, the new cam, and im searching for 391's and a stall converter. also how many inch converter should i run?