A 427 crankshaft and a 390 crankshaft are the same stroke. So if you use a 427 crank in a 390 block, you have a ......390.
Depending on your budget, you can either just use a factory 390 block, crank, rods, new pistons, etc., and have your 400hp.
For a little more money, you can use a new Scat stroker crank, rods, pistons, etc., and have about 445ci.
None of these cars really get good gas mileage at all, and you probably wouldn't notice much difference between the engine sizes that you have mentioned.
You might wanna tell us how much you wanna spend, and then we can guide you with that.