Mr. D,
I have to raise my hand; I have a 385-series with an undercar exhaust:
It's a ~2000 Everett Morrison. I'd guess every maker and also from year to year the shape of the headers can change enough to prevent them from fitting.
Sadly, when they started leaking, E-M (when they were in Tampa) rewelded them without bolting them to a head or a flat plate and warped them beyond redemption.
The car now has custom headers built by Sanderson which cost over $1700 ten years ago, modeled after the E-M headers - and they didn't fit! Sanderson admitted they'd never tried to bolt them onto a head
It took a local 8-hour rework to get them correctly installed.
Oh, I did try to fit various ready-made block-hugger headers along the way. No joy. YMMV!
Having said that, you will probably succeed at finding/making a set if you take your time and apply some forethought and lots of measurements! Then again, my automotive experience frequently seems to include getting things wrapped around the axle...
When you are done, pictures, please!
Tom