Ok going out on a limb, and a long post!
Stock j heads flow around 200 cfm@ at a max of 500 lift. cut a 2.02 in and a good valve job and it will flow in the 230s to 240@500 lift (600 lift the flow slows down(225 cfm))
I took a set of j head just like yours and did all kinds of porting, find water jacket, fill in with epoxy, ground some more. Filled in the floor, shaped the Short Side Radius(SSR) a hundred difference was.
Even got it to flow 240 by .300 lift and 270 by .500 lift! The problem, COULD NOT GET IT TO FLOW PAST .500 LIFT. Every time it would crash in flow, and sometime, the turbulence was so bad that it sounded like i was pouring water into the port. I finally fixed the problem! it was in the combustion chamber! USED CLAY TO EMULATE A CLOSED CHAMBER AND WOLA!!!! IT FLOWED TO .700 LIFT. You can't weld up a combustion chamber on a cast iron head. Finally did get a shape in the combustion chamber that would hold on past .500 but was Alway Noise!(these head were going to be for my 408!)
Ended up buying some 2.02 RHS head from brian at IMM (cast iron head with a closed chamber design) and of course, had to give them a quick flow before i put them on.
Flowed more at .500, .550 and .600 and held the cfm to .700!!! but what made me know i had made a good move was how smooth the air sounded going thru the port.(my velosite prob said so too!) Even at .700 lift.
Now Why did i tell you all of this? Your port might be going turbulent above .500 lift! (old cam never saw it)
Flow may be slowing down above .500 lift causing the head to produce less flow not more and acting like a smaller cam, not bigger??????
Like i said, going out on a limb......................
A couple more thoughts.
You probably could use a bigger carb, BUT the Larger cam and Same carb, AT THE VERY LEAST, would keep the same mph not loose 4!
I higher stall may get you a better ET but, both cams 60' similar so, you're making similar HP at 3000 rpm.
