OK, I'm dumb, need help

Consider that more horsepower requires more cubes, more rpm, or more airflow With a given engine size, you only have rpm and airflow to work with. Since the slanty is severely limited in the airflow department;
about the only way a normally aspirated slanty can make significant power is to rev it. So consider how much higher you want to/can, rev it, then select the cam accordingly. That will give you an idea of it's power potential. Everything else you do to her will be to support that cam, so that it can get close to it's potential. And then you will have to match the new higher resulting powerband to the vehicle, with different gears, and a looser TC.
In the end you will have a lot of money tied up in what? Something like maybe an increase of 45hp at 5000rpm. How much does your loaded vehicle weigh? 3600 pounds? That comes to an increase of; 45/3600=.0122hp per pound........ and now the power peak is at ~5000 instead of 4000 . If you started with 125 hp, now you have decreased your loading from 3600/125=28.8 pounds per hp to 3600/170=21.18 pounds per hp. Hallelujah. Ok I get that you want to be different, to be unique, but maybe there's a reason almost nobody hops up a slanty for duty in a heavy vehicle.
Now in a lightweight-A, you might get 2800/170=16.47pds /hp...... but you started at 2800/125=22.4 about the same as the 3600/170=21.18pds per hp, from above.
If you want to have some real exciting street fun. I recommend 8 to 8.5 pounds per hp.
If you just want to relive your youth with like the memory of a 340Swinger, 10pds per hp
If you just want to squawk the tires now and then 13 will do it.
IDK what 21.18 will do for you, but ....... I don't see a line-up begging for a ride.

Ok but how about super charging? See now we're thinking. It takes about 7 psi to double your power, and I heard the slanty will take 10 to 15 or more psi. Lets pick 12psi. So that's like (12/7x125) +125= 339hp. Lessee that comes to 3600/339=10.62 pounds per hp, right in the fun zone.
And you can leave whatever gear ratio that's in there alone. And you don't need 5 gears anymore. The A833 will take anything you can dish to it. And if you wanted to, you could even run the A833od. (Or a stout 904 with V8 guts.)
But if a manual, then you are gonna need a V8 clutch and a V8 rearend,with a SureGrip, and some serious traction control. But since you now already have 2/3rds of the V8 stuff............. just how unique are you?
you could get similar results with waaaaaaaaaaay less money spent just by adding a couple more cylinders.
Jus trying to help, not ragging on the reliable as a stone-axe, six.