259/264 duration @ .050?
I'd like to see more exhaust duration myself and the bigger headers. But that is a decent sized cam that won't wake up till later in the RPM band like you exp. now.
If the heads stop flowing @ .500, I'd say there not ported or not well pocket ported for RPM purposes. (Maybe the shop new they were not ported or lightly ported) Most J heads I have read or talked about stall @ or around .500 @ 200 - 220 cfm. (Give or take. Flow bench dependent?)
Work on the launch traction issue.
A dual plane (RPM) will add torque, but not in bucket loads. A smaller carb would help launch and mid range. (750) The air fuel velocity would improve.