Headers are designed to fit a stock car with a stock body. So if you have a stock 350 (using any factory head other than a Vortec head) it will be the same as the 283 that was installed in your car in the year it was made and headers will fit. The Vortec has the cast iron exhaust manifolds located in the original position but the ports themselves are raised and reshaped such that it is very hard for standard tube headers to seal the top of the port. As such you will be constantly fighting exhausts leaks.
They make a special header for Vortec heads to fit in Camaro bodies and that may fit. BUT you have an X-frame car and the rails neck in toward the center of the car contacting many headers not specifically built for a 1958-'64 Chevy full size car. If you buy a set to try first verify the conditions that will allow you to return them. That usually means no scratches or dings and dents in the tubes. Also expect to pay freight back and the possibility of a restocking fee.
I have not bough an off the shelf header in decades for myself. All of my projects have had a custom stainless steel header welded up by a local muffler shop from a weld together header kit. (I can weld mild steel and fabricate larger pieces; but I am big, and I am not a surgeon, nor do I have that good a foot, hand, eye, torch control to TIG weld headers with Carbon-dioxide or Argon gas flooding the tubes.)
These custom built headers will fit my car when they are done, and the cost is less than buying a custom mail order header from Lemons or similar companies (still averages around $1,200 a set). Customer's cars I try and fit what they bring into my shop AFTER I have first read the footnotes published on line by the header manufacturer that tells you what accessories (power steering, Air Conditioning, AIR pumps), or angled spark plugs, or transmission combinations that will not fit the one size fits all cars off the shelf header.
They also list any modifications you have to make to the headers to make them fit (such as partially collapsing tube number # to clear a suspension part). It makes interesting reading, but I have discovered that most of my old customers could not read fine print. I like to point out these details and print out the foot note page and staple it to the work order just so that the customer knows what to expect with the install. (If not Mad Hatter Muffler shop was just down the street and they charge less for mild steel header kits).
I suggest you down load the on-line catalog found on each manufactures web site and read up on what their headers will and will not fit. Then contact Summit, Jegs, Speedway, or any other mail order discount speed shop as to what their return policy is before you buy anything.