Horsepower estimate?

In 1974; the bone stock 8/1 4bbl 360 with log exhausts,and 268 cam was rated at 240 NET hp. Some say to make that into crank HP, you can gross it up by 1/.85, so; ........ 240/.85= 282 .......... box-stock.
Now, IDK how accurate any of those numbers are, but I also know that the 5.2Magnum is rated 230 NET, with a lot less cam.
So now; add say a one-size bigger cam,headers and a full length 2.5" exhaust, and some decent ignition timing; Ima thinking that adds up to more than just 20hp crank. But I'm not willing to bet just how much more,lol....
IMO
You have close to the right engine package for street right now. Concentrate on gears and stall, and change the tune for them as may be required..

BTW
For a streeter, the 650 is about the right size, more or less. On the street you will not notice much difference with a bigger carb, until after about 4500rpm, probably in second gear (cuz you will be burning right thru first), so with 3.23s and 25.5" tires, that will be ~70 mph. With 2.76s, 4500 will be ~80 mph