A pic might help.
I know on my 460 that 31512 is the headpipe to muffler gasket (x2) and there's nothing but a ball and socket joint at the cast manifold outlets.
A loose flange on the head pipe draws the flared end of the pipe tight up onto the casting.
You can see that there is no flat gasket face if you look up a replacement manifold on eBay or whatever vendor site you choose.
(This is RA for a 1985 drivers side application)
Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with a Holley 80508 and Performer intake.
Too much other stuff to mention.