383 going in a dart
I went to big block .com and looked into there tech section and read up on how to put regular "B" body headers into an "A" body. I had on hand Headman Elites which are the same as there standard header. Only a thicker flange and coated.
The instructions at BBD.com were good, but they did miss one thing.
I beileve it to be alot easier if you take out the steering column and trial fit it in with the engine fitted in and ready to be bolted down. But dont do it yet. Also remove the steering linkages. This makes header fitment alot faster into the body. From there, you jack the car up and onto jack stands (After steering column fitment is done.) as high as you can get for your own comfort and ease of hammer swing to fine tune the fitment of the tubes under the car when you reinstall the steering linkages.
Been there done that just 2 weeks ago.