I'm building the car to take way more power than I actually have at the moment. "Plan for future growth" is really good advice.
Buy once, cry once as they say. Lol.
It's certainly true that with certain rock solid goals, there are cheaper ways to accomplish the task. 8V with a big turbo. 16V with a turbo. V8 with no boost. V8 with a little boost. V8 with all the boosts.
You just have to be honest with yourself and think if your goals will really remain static or not.
Although from a practical standpoint, at least on street tires, going up in HP from about 300 only makes the car faster in higher gears, you're just going to be traction limited until higher and higher speeds. But still, having 4th gear pull harder than 1st gear used to is pretty intoxicating.
This discussion is a little off the rails of the OP's original question. But since we're at it, I will say this, for a friendly counterpoint:
You can have other goals for a build besides just power. Many of us go with a V8 not so much for speed but for silence, comfort, smoothness, flexibility. The 4 cylinder redblocks all suck for NVH. Their driveability with stock turbos is good, but the bigger the turbo, the worse it gets. A naturally aspirated V8 will produce the same power at generally similar cost but the operating characteristics are much calmer and won't wear you down after a couple hours.
Making decisions that accommodate more future growth is always a good idea, sure, provided they are within realistic context. Running a clutch that has a little extra capacity, a radiator that can exhaust a little more heat beyond what you're making now. But inevitably there are tradeoffs, either in cost or functionality, for building something that is capable of performance way beyond the present envelope. Wide tires mean tramlining on the highway. Having to swap to a heavier rear axle means worse wheel hop on broken pavement, and more difficult wheel options entirely aside from the cost and effort of doing it. Bigger driveshaft, etc, etc. Need serious, serious brake and suspension changes to be safe with major power increase. The car is a system, not a sum of parts, and to deal with 700hp in a '60s body shell originally designed to have a B18 under the hood
there is a LOT of work to do. I disagree that everyone needs to have a Hellcat level ultimate outcome in mind with every decision they make. All for not being shortsighted and planning for what you really want in the end, but there's also something to be said for optimizing for something more suited to the actual task in mind as long as that task is well developed -- not necessarily always the full nuclear scenario. Unless you really are gonna go for 700hp or more in the end, heavier and tougher parts are not only unnecessary and expensive, but can also make the car worse to live with.
Me, I cannot imagine wanting more than a few hundred hp in an old Volvo. I've had that much and it was fun, but enough. Can't put the power down until 4th gear? That's way over 100mph. When and how often can you do that outside of a racetrack? And in an antique car with no modern safety equipment and no rollcage or helmet?? Devil's advocate, but they are serious questions.
I will take something that feels good in real world driving any day at 1/5th the cost and effort of something that can break the tires loose at triple digit speeds. There's a place for swaps that bolt together with minimal cost and effort and deliver a nice, snappy-feeling product within the bounds of what can be used on a public road. YMMV, but I don't think everyone reading on here and planning their builds needs to shoot the moon with every single component choice.