I think you need to take a deep breath. You found the source of the vacuum leak. Replace upper intake and then if the problem is not resolved, make a decision. You are about to spend a lot of time and money needlessly IMO, but it's your car.
Getting rid of the computer controlled wires, but keeping the gauges and lighting wires in the front harness, then adding back the wires needed for a different ignition system without making a fire hazard is a big step. I would see if a DUI HEI style distributor would your carb manifold to keep it simple. But only if I was that mad at my car.
And that would make sense,as aluminum expands w/ heat.
Summit will completely take care of you on that, but if you are trying to justify a bad intake manifold as typical as an excuse to convert, you're kidding yourself.
If an EFI intake was prone to failure, there'd be about 73 bagillion threads to support it.
You know now what the problem is. Simple fix. Buy a new one, Summit will credit you out for the one you return, and you'll be able to join the ranks of the belly button hoarde of happy fuel injected 5.0 owners.
Yeah you guys are all right about keeping the efi.. I just got stupid mad about a bad part. No ones fault. It just happens from time to time. I'm sticking with the efi.