Don't know the answer but, I have a brand new line for my 70 Duster that if I'm not able to straighten out the tittle, I'll be selling. That and a bunch of other new parts I bought for it like brakes , suspension and other parts.
I have run that pump on several cars over the years including my now 69 360 Dart. I’ve never had a sealing problem. But I-always ran a short section of rubber hose on the intake side and output to keep vibration down. I just use Teflon tape.
I went from a 1406 Edelbrock that I ran on my 360 Dart to a 1906 Edelbrock AVS2 650. Very pleased with the difference. Throttle response is and cruise throttle response is much crisper. This would get my vote.
Start with the simple test 1-check fuel filter 2- check for accelerator pump shot. With the engine off, remove air filter . Open the choke blade, look down the primaries, slowly open the throttle and look for pump squirting gas. Go from there.
I just installed a 1906 650 AVS2 Edelbrock on my 69 360 Dart. Ihad previously had a 1406 600 Edelbrock abd before that a 4777 650 Holley DP. The new AVS is by far the best performing carb I’ve had on it. Very resonsive and starts much easier.
I have a 68 filler neck and it is exactly like the one in my 69. I just replaced the tank in my 69 Dart and it I identical to the One I took out of a 68. Vent line runs along the back of the filler tube then passes thru the floor pan gasket and ends up in a hole in the frame rail.