340 advice

There are lots of off the shelf headers for your application.
There is no need to buy custom.
Maybe go to Summit and type in the search box for small block headers in a '65 B-Body and you will get a lot of options to choose from.
It's a lot harder with A-Bodies, because there are only a couple of brands that fit well without a lot of fooling around.
Perhaps you could buy TTI's or Doug's and use them now and when you are ready to sell the combination, A-Body guys would buy the whole package from you because it'll slip right in with no problems.