I have used FAST XFI and Holley Dominator EFI systems before. Recommend you consider the following:
How much manifold vacuum does that 502 have at idle?
If it is less than 10"Hg, I would be cautious about using the EZ self learning systems as they tend to not provide optimum fuel maps for low vacuum applications. The XFI can handle it easy.
Are you running manual or automatic tranny?
Some self learning systems tend to keep relearning with manual transmissions as you go through the gears, not optimum.
Are you comfortable using a laptop to tune EFI systems?
I had a bad experience with my first EFI system because I lacked training on tuning. Tech support didn't help that much. I ended up attending EFI University so I could drive my car.

Whichever way you go, I strongly recommend you read the very top post on CPGNation, FAST Support Forum, "before you buy any self learning EFI system, please read!"
That post applies generically to self learning systems, not just FAST.

Best of luck and keep us posted so we can help going forward.

I'd personally have to ask you why you'd think EFI is going to make the car run better --- if it isn't tuned well now -- what do you think the EFI is going to fix??

I'm not saying NOT to go EFI -- I'm a big EFI lover...... but I wouldn't expect a poorly tuned motor to run any better with EFI than you could tune it with a carb.

And here's the real debacle -- and I'm NOT bagging on you -- but if the motor isn't running well --- all the time -- then what are you attempting to fix??