Bad way to start in my opinion. Cheapest price means cheapest construction.
Depends on application. A street/strip motor up to and including 355 would do well with 1 5/8" primaries. 383 and larger, depending on rpm's, might do better with 1 3/4" primaries. Hedman makes a nice header. If you were using a very large motor and buzzing it, you might opt for 1 7/8" headers such as Hooker Super Comps. The reason you don't normally see pipes bigger that this offered for a small block is maybe because they don't work.
What's the motor size and what is the intended use of the vehicle?
Ha Ha, good luck finding any pre made header to fit, you are looking at having to go custom unless you can get lucky and find something for a different application that might happen to fit or fit with just a few mods. Take a look at Schoenfeld Headers, maybe you can use or slightly rework a Modified or Sprint header, I always give them a look and have used them a few times.
I used Schoenfeld SBC Street Stock 1 7/8" for a '78-87 GM metric chassis when I did my mini-tubbed 10.5" Radial Vega, I let the header decide where the engine had to sit, still ended up with 6 3/4" engine set back but wanted 8". I wasn't going to spend $700+ for custom headers, these were $179.
Getting ready to do a Straight axle Gasser w/ a Vega body and I'm going to let a set of IMCA Modified over-the-frame side exit headers decide engine location for this one too. It was the only way to find a cheap fenderwell header.