The 70's affected all cars....
A 455 surely has the foundation to make some power, you just have to coerce it out. A cylinder head swap and a modern camshaft would help tremendously.
A compression ratio bump will also help. I do agree with Bear though, the difference in horsepower between a 9:1 engine and a 10:1 engine is negligible and if you live in an area that has poor quality gasoline or only low octane gas available, it's best to err on the side of safety.
Flat tappet cams are more difficult to mess with these days and I generally try to talk my customers into hydraulic or solid rollers. However, when the need arises, I will do them with a nitrided core, EDM lifters, and break in the camshaft with Brad Penn oil and reduced spring pressures. I'm building a 400 for myself right now and I'm using a custom Comp solid flat tappet...with some Crower EDM lifters. It will get beehive valve springs but will be broken in with some low pressure outer springs from a dual spring package.
To the OP, an aluminum cylinder head swap can get pricey, but when you look at it in relation to R&R'ing your factory heads (and some port work), then it's very cost effective. With any engine, the heads and camshaft make the power and there's no reason why you can't make 1 hp/ci with a little bit of work.