I would liked to have had a 427 in my Cobra, but Sideoilers are hard to find and very expensive. Additionally, a center oiler 427 looks like a 428, but has the problem all 427s have.....max recommended overbore is 0.030", and most will already have this done, so that's not good.
The other original big block engine was the 428 and are easier to find. I did up an overbored 410 (to the point I call it a 428), but pistons arn't the easiest or cheapest to find. A full overbore to a 428 is risky with most 390s due to cylinder wall thickness, however some 390s, especially later year 390s do have thicker cylinder walls allowing for a 428 overbore.
A 390 with headers, carb and especially a cam change can easily be made more powerful than a stock 428, and looks the same from the outside.
The thing about horsepower, is that it's work done over time. It's hard to develop full horsepower from any of these engines due to the car being so light, except at very high speeds, which is extremely unsafe, not to mention illegal on public roads, so as someone mentioned above, a nice torquey engine is the way to go, and that is mostly dictated by camshaft selection (although a 428 crank obviously helps).
As far as gas mileage, don't worry about it. Not only is gas the cheapest expense of owning a car (compared to purchase price, yearly registration, insurance, etc.) it is unlikely that you will put high mileage on a hobby car, although some peple do I guess. So your total gas expense per year may not be a lot.
Most of the fun is in second gear shifting to third....but gas mileage will be bad when you accelerate hard....like Niagra Falls in the carb(s). But highway cruising isn't bad. Light load, better fuel economy.
I find taking the Cobra for a drive a major event. I have to walk 100 yards to my garage, open the garage door, start the engine and back out. Then I have to keep it idling for a few minutes so it won't stall (no choke) and then drive it at least 5 miles until the
oil temp starts to come up before I dare put my foot into it. Once back in the garage, I like to wait until the electric fan automatically shuts off before I leave, not to mention opening the hood to check for leaks (like gasoline).
Almost embarassing, I ordered my kit in 93, picked it up in 94, got it done and on the road in 96, and it just turned over 5000 miles a few days ago. Lazy? Maybe ...OK, yes I am. However, I must admit, I enjoyed the building more than driving it. I used to be embarrassed by the looks I got, but finally realized they are looking at the car, not me. Although one time, I was cruising past the local kids hangout by the Liquor Store at the mall one Saturday evening, and one young fellow hollered "Hey, Old Man!" at me. Guess the gray hair prompted that. My GF sitting next to me had a coniption fit laughing so hard.
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