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IIRC the A12 6Pak fit under my '65 Coronet hood with no problem. The drop style '70 air cleaner would be even easier.

Old school, CH4B, Torker, TM7 all will fit. Add a drop style air cleaner and plenty of space.

I always took them out, one more un-needed part.

Gas has been an issue as long as I can recall. In the mid '70s for my drag car, I mixed AV 100 gas 50/50 with my 12.4 CR motor. Ran great. Even when they went to LL 100, worked great. I even tried the purple military 140 (helicopter) stuff. AV gas can help.

Back in the day, mid '70's I mixed 100LL with the premium available with 12.3 CR. It worked great. With todays non oxy, I'm sure 100LL 50/50 would work fine with iron heads and 10.5 to 11 CR.

Not what sure the model number was, it was a Holley Ultra XP.

I have gone to a 1050 Dominator. It definitely makes more power, and I haven't done any tuning on it. I still am amazed the Holley 950 we tried didn't do better.

That is not a '69 1/2 style air cleaner. I have no idea what scoop you have or what location it is. I am not an A 12(69 1/2) expert by any means, but I did have a partially OEM A 12 setup.

You do realize you have a bunch of mismatched parts.

Maybe it wants a little more idle opening OR a little more static timing. Just my guess.

A loose P/V would cause much of your problem. Hope you can make those end carbs work for you.

Keep in mind the OP does not have OEM end carbs. The manual and likely the bulletins probably don't apply to the aftermarket mechanical end carbs.

We dyno'd my 512 with Indy Max Wedge heads with my 850 DP, 692 HP, with a trick Holley 950 694(no tuning), a 1150 Dominator (no tuning) 696 HP. Overall height is an issue.

Not sure which mech end carbs you have. IIRC there were several versions available. The float bowls and floats are very different than the OEM vacuum end carbs. The only time I had the deep needle/seat & nut issue was with a non Holley needle & seat with the very thick brass adjuster nut. The...

I agree with Bob, you have symptoms of a ruptured power valve. Try another power valve. The 6.5 should be good with a fairly mild cam.

The backfire may have broken the power valve. Normally looks like float level. Try opening the end carbs a tiny bit. But I have no experience with the mechanical end carbs.

Back in the day, I ran about 50/50 low lead 100 AV gas & the premium of the mid '70's in my drag car, 12.0/CR. The 100 LL will be fine on the valves.

You don't listen to advice, try to get lucky.

Look at post #5 for possibilities.

I agree with pnora, the shooter size is not the problem. Shooter size affects the response after the car is running, and getting it moving.