I have 2457's on a 69 with a 496. They fit my rig perfectly though I don't have PS. No mods necessary at all and for BB headers, that says a lot. Pretty simple install; no PS, no A/C, 4-speed car, lock-out removed. I have AFR heads on this car and I probabaly had to grind on the flanges to clear the lower head bolts.
There's a good 5/8" clearance between the tube and my manual steering box so I'm thinking I'd have clearance if I had PS on this car.
If you're having clearance issues with the PS box, use these headers instead:
Click on the on the photo and you can see how the number 5 tube is routed over the number 7 which provides all the clearance you need around the PS box. I use these headers on another 496 car with PS. But for these, I had to do some re-routing of the fuel line. No big deal.
As far as grinding flanges to clear the lower head bolts, that's a problem you'll likely encounter with any headers with aftermarket heads. Aftermarket heads and their bolts (or worse, studs) will cause clearance issues. Again no big deal. Simply mock up the headers, mark with a Sharpie the areas that need to be ground a bit, and go.
My sense is, many ask too much of the header manafucturers when it comes BB's. If you're running a BB, some adjustments to your new headers will almost always have to take place; even though you have the correct frame and engine mounts and the usually forgotten tranny crossmember. The tranny crossmember comes into play yet people forget; the correct BB crossmembers will move the rear of the engine toward the passenger's side of the car, away from the PS box.
Part of the fun of BB's. It's a huge difference vs. SB headers with 1-5/8" tubes.
You're playing in a different sand box now.