How to tune a freshly built engine?

As far as I know the computer of the car decides how much output your engine has based on the air injection, fuel injection, timing, etc.
So if you build an engine for high power output how do you have it run for what you built it for?
Let's say I take a Ford 5.8L Windsor. I put aftermarket heads, headers, exhaust, cams, forged Pistons, rods, rings, cams, intake, and all that and make it in a way that it can handle 300-400 hp while stock it puts out only 215.
It wouldn't work if I put it back into the car without having the computer provide the engine with what it needs to put out the power it can.
So what procedure would I have to take?

I'm NOT ANSWERING! I JUST WANT POINTS

You buy an aftermarket hi-po chip for the computer.

Sorry - I'm out-of-date and don't know what's available - gosh, when did they stop making Windsors? Do they still sell crate engines - and what do they come with… Better ask you speed shop.

If you want a computer-ized engine & go with fuel injection, you'll need an aftermarket fuel injection computer to handle the increased fuel loading. That 'computer' will be programmable & can watch & change your engine parameters, like timing & injector cycle times. You can program it for a lower power output by limiting the timing or the injector cycle times, or tune it for the maximum output, which would best be done on a dynamometer.

I don't think the 351w had a 'computer' like you're talking about.
Maybe I'm wrong.