Re: 1971 Chevelle SBC to a BBC
Tom, I did this swap in my 71. Well I should say I did a sbc to bbc swap but I used a 402 and went from a 307.
Anyway I used hedman 1 3/4" painted steel headers. They fit like a glove you can drop them both in from the top if you wanted to. The disappointment here was the non hi temp paint that they come painted with. The mess it made when my fresh paint burned off was disconcerting. These hedmans came with a ball and socket flange and a 2 1/4" collector outlet. Header mfg should be able to answer your port to flange question.
If you had 350 originally in that car then 350-454 all use the same frame mount and motor mount combination. The combinations are "tall and narrow" or "short and wide"... something like that anyway.
I bought a new starter. I had serious heat soak problems. Even when cold it would crank slow. Maybe the 307 starter was intended for low compression motor or maybe it was tired.
Good luck man.
1971 Chevelle Malibu "400" Convertible
Cottonwood green/white top/bench/column