What are you planning on doing with the car is the main question. There are several people dragging etc and are using the 4100 carb. Imo if you aren't going to be doing any heavy dragging or racing of some type the 4100 will be fine. I have only experienced one Holley and it was a nightmare. I have heard, like you, that Edelbrock is more user friendly. I chunked the Holley and got me another 4100. It's almost plug n play and I don't do anything but drive around so it works great for me.
Not sure what mileage improvements if any, you can get out of a 390. What do you get now? Way back when, with my original engine and carb I got around 16mpg. Now with a bit less than stock I get 9.

Deb