You have been misinformed about availability, but doubtless it is more difficult on your side of the Atlantic. These log manifolds come up pretty frequently, and are usually cheap, so persuading someone to ship it to you and the expense of doing so would be your main obstacle. There is a forum that deals with vehicles with our engines (and I am assuming that yours still has its stock Wide Block Poly 318 in it). Don't be offended to find that it is a pickup truck forum:
http://www.dodgesweptline.org/phorum/index.php
I would post a wanted ad in their classifieds section:
DTAForums :: Buy/Sell/Wanted
And I would certainly contact this chap just in case:
Poly 318 Stock Exhaust Manifolds...
Even though he's likely sold them, he may know of more. The type of people on these forums tend to be very honest but prickly by nature. My approach with them is to trust them completely and send the money up front. I have never been cheated. If you proceed with normal British caution, these fellows take offense and go away.
This is what your manifolds should look like if you have the stock engine: