When I build a street engine for someone the time spent sort of runs like this, in order of time spent
1 - Determining what they want with all kinds of questions, opinions, examples, and discussion. Time spent up front is worth it's weight in gold.
2 - Waiting for parts
3 - Waiting for machine work (of course these two don't matter much, but it does go into expectation management and is worse since COVID)
4 - Building and documenting the engine (goes pretty quickly)
You don't have to decide before you talk to the builder, unless you ARE the builder LOL. However, I think the RPM range should be your guide more than the HP peak. Once you lock that in, which I actually think you did in your first post, then it's really budget more than anything.
One thing is I would not do is compare your Cummins 400 HP to a gas vehicle, you won't have to build boost, rotating assembly is lighter, every single component across your car has less parasitic loss and no straight six winds up like a V-8 ( flame suit on but I won't budge on that one). That being said, Jay's advice for a blower is a neat idea, very neat idea, but I would say that it requires a fresh engine underneath it anyway. I think an older rebuild or stock would quickly start pressurizing the pan and also start finding the weakest links.
If you are doing a 445 or 462, or even bigger, it's pretty straightforward to build a mid 500s HP engine with no significant maintenance and pump gas. It's all about using the right stuff. Heck, you can get there without fancy stuff, my own F100 is 490 HP / 550 TQ with a 15 year old custom flat tappet, ported truck heads, a port matched Victor intake and 9.75:1 compression. It peaks at 5000. I may not wrangle with a pumped Cummins in the mountains with 5K on the bumper, but gets your attention is a short box lifted 4x4! I filled it with 89 octane yesterday
Swap that to an RPM and a modern HR cam, you are in the 500s, add better heads, it gets more exciting.
I would take the time to figure out exactly what your budget will handle, (and if the car carries that value) be sure to think about some sort of posi, building a tranny to take the power, what you think about the current gear ratio at the speeds you run most frequently, etc, and then what you want to spend on an engine. Once you get there, there is sort of a box everything fits into, and then the parts maximize the power to fit in that box.
I do think you are right to look at some of the street builds in the dyno section, Brent's truck motor you referenced was stout, as were many more of the pump gas motors on that page.
If it were mine, and assuming a 3.00-3.25 gear and a stock-ish tire size, my gut says 445 (unless you have a 428 block) and just build for a 5500-5700-ish peak with a set of Trick Flows and an RPM intake, then build the supporting parts around it. If the wallet screams now, wait or back off from there
