Hi there & welcome to the forum!
The one significant factor in holding back an Olds 403 in a ‘bird is compression, namely the lack of it.
Just under 8:1 CR is too low to take full advantage of any gains from a cam & or intake upgrade.
This doesn’t necessarily mean ripping the block apart & fitting flat top pistons, although that would be the ideal solution.
Achieving 1 & a half-point increase in compression is quickly done by installing 350 small chamber heads early or, even better, a pair of aftermarket aluminum heads, Edelbrock, etc.
The main factors are, of course, how much investment is compared to achieve the best results & that’s all dependant on what you are hoping for.
Better quarter times?