318 Spread Bore No Idle

[JedIEG]

I really hope it's not an intake leak... the motor is in good condition and I know about all but the first 7k miles of its existence and heads and block have not been milled. I won't rule out a fitment issue causing a leak though, the intake is an old edelbrock performer 318-360. I'm thinking to pull the valve covers and rocker shafts off to close all the valves, make a cover plate and pressurize the intake to with compressed air to listen for leaks. 15 psi would seem like as much pressure difference as the motor vacuum. I'm worried about hurting the stem seals though. Any other thoughts on diagnosis options?
On the other hand, I am trying to figure out how to eliminate the accelerator pump transfer tube in the 4175 that limits the metering block choice. I have a piece of 1/4 aluminum rod I have drilled to replace the tube and cut the be flush with the body-metering block mounting face. I am thinking to JB weld the outside of the tube in place so I can use my 4160 gaskets and metering block. Anyone have experience JB weld holding up to fuel?

 

[JedIEG]

Finally got some time this weekend to look into the suggested issues.
Intake side: when testing the engine I got the carb to backfire and noticed smoke coming out of the breather. I took the manifold off and regasketed with now FelPro Printoseal gaskets for the 318 heads . There were no alignment issues with the manifold this time.
I refired after letting the RTV harden for 24hours. The engine is shutting off just as before. I bought a vacuum gauge and hocked it up to the rear port on the crab throttle plate. When firing, the engine is running between 10-15 in-Hg vacuum, so I do not believe there is a vacuum leak can be the assignable cause. I have tried both metering blocks on the 4175 with no difference in results. I know there is fuel in the bowl based on the amount that drains out when I undo the bowl screws.
I am going to try to reassembly the 4160 I have on the shelf to see if it is a main body issue with the idle circuit. Otherwise, I am at a loss.

 

[JedIEG]

For completions sake, the issue was the 4175. The engine is running with the 4160 on it.