Chris(NJ)'s, where am I going with this, already built build

[Chris(NJ)]

Did the gasket tonight. I didn't have to drop the tank, so that was nice. Took out the 5 screws and realized I was pretty close. So I wound up unbolting the driver side mounting bolts for the tank which dropped it enough that I was able to get the sending unit out far enough to get the new gasket over the top. I'm sure there must be some junk floating in the tank. Even as I carefully took out the broken pieces of gasket, there was not nearly enough to re-create a full gasket. Anyway, simple fix. Took me an hour. The majority of the time spent figuring out the stupid bolt hole configuration was oriented correctly.

Now I need to find out why the truck is running so poorly all of a sudden. About a month ago I started getting horrible gas mileage (Holley 600 carb). Like 8mpg, mixed driving. Now it's dying out on me too after a cold start and once I put it in gear. When it's warmed up and I come to a stop, it stumbles like crazy but doesnt die. And it also has a serious dead spot coming off idle. When I give it gas, it takes a couple seconds to kick in.
I don't know how you guys do it with carbs...it's driving me nuts lol. I want to go to an FI motor w/ an ecu that tells me whats wrong when it's not running right