You are welcome to poke around on my web site (link below). There's examples of two different front ends with a big block. I recommend one of the newer style front clip options which make it an easy project without any or little fabrication. The most common are probably Chassisworks, TCI, and Heidt's, but there a many others too. There are plenty of guys on this board with a wide array of different vendors products that can give you feedback.
I have asked the same question too.. I purchased a 67 with heidts and put a bbc in it.. I am to the point now where i need headers and so far its either get some from lemmons or make some custom one myself..because I want in frame headers. I have noticed if you buy a alston front clip they make thier own bbc inframe headers, whereas heidts does not.. so thats my opinion..
I have a Chassisworks clip with BB & their headers that fit perfectly but not sure if they would on someone else's or not. I know there's several others on here that have Heidt's clips with other vendors headers that seem to have worked, so hopefully they will chime in with their input. I think you have other options than Lemmons or U fab.
Your semi-fenderwell headers look similar to mine except I have two tubes that go through the fenderwell instead of one. I used to a have a set of Hooker Super Comps on my Elky that had one tube like yours though. How big are the primaries and the collectors?