Advice for a ~450HP 351w build

Hi all, I wanted to get some advice about building a 351w going into a 1969 Mustang for the street/weekend driver that could make around 450HP - it doesn't necessarily have to be at the low end.

Here are some of the goals I'd like to hit with this engine:
-Stock 351w displacment (avoid boring or stroking if possible)
-7k max RPM
-Makes around 450HP (I'm not too worried about torque figures)
-Pretty decent fuel economy (At least 10mpg, preferably 15+mpg)
-(Optional) A stock sounding idle rather than a lumpy big cam idle

I'm a complete noob at engines, so some terminology may be lost on me. I'd like to learn more and build my own engine rather than outsourcing an engine builder or buying a crate motor. I'd plan to use an aftermarket block (Iron Eagle or Man O' War) and forged internals so that I don't need to worry about weaknesses with stock blocks or internals.

Is what I want to achieve realistically possible? I don't mind spending money if it means achieving the goals I'm aiming for. The primary idea is to have a stock style 351W that produces more power than originally from the factory, but has more horsepower and can rev a bit higher. If possible, I'd like to be able to maintain a decent fuel economy as I'm based in England, where fuel is a lot more expensive than in America. I'd rather trade some horsepower for better fuel economy if that's what it comes down to. I can appreciate this is a classic engine after all where fuel economy is not really a thing you consider, but I'd like to boost fuel economy where possible.

What would be the primary way to make more power if keeping stock displacement? Would it be better flowing heads, a more aggressive cam profile, a larger carb? I'd like to avoid increasing the displacement for the sake of originality and trying to decrease fuel consumption from having a bigger displacement.
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That's impressive, I didn't think you could make power so easily with a junkyard block. I take it the build had upgraded internals rather than stock though?

Spend your money in better places than an aftermarket block unless you're planning to stroke it out or boost it.
Where would you recommend reinvesting that money in the engine? Better flowing heads? I'd happily make more HP if it doesn't sacrifice fuel economy, but I have no idea what point the trade-off is.
Anything making any kind of power without the advantages of modern fuel injection, variable valve timing, and engine geometry is going to drink gas like a drunk drinks booze. The 308cid engine making approximately 320-340hp at the crankshaft in my Mustang II (it made 267rwhp on the chassis dyno, so I'm assuming an approximate 20% loss through the slushbox and driveshaft) only got about 10-12mpg with a carburetor and about 1mpg better with throttle-body fuel injection, because, at it's heart, it's still an engine from the 1960s with a big cam, high flowing heads and intake, and driven hard anywhere and everywhere. Of course, the gearing in that car doesn't help, you could run something more conservative than the 4.10s I'm running and do quite a bit better (you'd have to match the gearing to the powerband of the engine for it to be a pleasant driver).
Ah, that’s kind of a shame. I guess if it’s still above 10mpg with city driving that’s not too bad. The plan would be to mate the engine to a Tremec T56 6-speed, both for the 6th gear comfort/fuel efficiency on highways and a T5 most likely would not handle the torque as they’re rated for maximum 350lb/ft torque. Maybe having the 6 speed with a wider ratio would help out in some way? What would be the best way to match the transmission to the engine’s powerband? How do you alter the powerband so it’s at the lower end, higher end, consistent throughout, etc?

So at the end of the day, I might as well go for HP seeing as I won’t salvage any fuel efficiency? :)